that an intrauterine contraceptive device may be inserted only up to five days after unprotected intercourse will restrict unnecessarily the use of this highly effective postcoital method. It is accepted clinical practice to fit the device up to five days after the calculated earliest date ofovulation.2 In emergencies where coital exposure took place more than five days previously, the shortest cycle length ever experienced by the woman must be ascertained and the day of ovulation calculated. An intrauterine contraceptive device can then be fitted up to five days after the earliest calculated date of ovulation. In a normal 28 day cycle this extends the time allowed for use of the device to day 19 of the cycle. This is of particular relevance when multiple exposures have occurred. S RANDALL Department of Family Planniing, St Mary's Hospital, Portsmouth A KUBBA St 'Fhomas's Hospital, London I Reader FC. Emergency contraception. BMJ 1991;302:801. (4 Mlay.) 2 (Guillebaud J. Contraception: your questions answered. Edinburgh: Churchill Livingstone, 1990:266. 3 Yuzpe A, Kubba A. Post-coital contraception. In: Filshie M,
Guillebaud J, eds. Contraception: science and practice. London: Butterworth. 1989:135.
SIR,-Dr Peter J M Davis expresses concern regarding the semantics used for describing postcoital contraception.' There would be more sympathy for his adoption of the "moral high ground" if his letter did not display numerous inconsistencies. There are many translations of the Hippocratic oath and the one chosen by Dr Davis strongly supports his views. He should, however, examine the other passages of this pre-Christian philosopher's thoughts on medical ethics and ask himself how many of the normal day to day activities that he accepts as part of his routine work in general practice are, in fact, proscribed by the oath. If Hippocrates's thoughts on confidentiality are accepted as sacrosanct then it would be impossible to abide by the notifiable diseases legislation; his admonitions to accept the wisdom of his teachers could prevent a practitioner ever applying to go on an updating course. Perhaps Dr Davis does follow that particular part of the oath and that is why he still believes that modern intrauterine contraceptives work predominantly by preventing implantation. (Their major effect is in preventing sperm ascent through the genital tract, thus reducing the number of fertilisations.2) I hope that Dr Davis's patients are fully informed of his views and therefore the limitations of the contraceptive "services" that he is actually prepared to provide before they sign any agreement. Hopefully the purchaser-provider philosophy will allow family health services authorities to clarify with family practitioners with such an attitude to contraception that they are offering only a limited service. Perhaps we should not be too hard on the likes of Dr Davis for displaying inconsistency. The Hippocratic texts themselves describe and advocate methods of abortion for certain pregnant females (slave prostitutes).' It was the Abortion Act 1967 that first legally enshrined this enthusiastically proclaimed right to exercise a conscience in treating patients and thus ignore another Hippocratic precept. DAVID R BROMHAM
Department of Obstetrics and (ivnaecology, St James's UniversitN Hospital, Leeds LS9 7TF I D)ais PJM. Emergency contraception. BMJ7 1991;302:1082-3. (4 May.) Sisin I. IUDs are contraceptives, not abortifacients: a comment
2
BMJ
VOLUME
302
25
MAY
1991
on research and belief. Studies in Famils Planting 1989;20: 355-9. 3 King H. Making a man: becoming human in earl5 Greek medicine. In: Dunstan GR, ed. The human embrvo: Aristotle and the,Arabic and European traditions. Exeter: University of Exeter Press, 1990: 10-9.
Early pregnancy assessment units SIR,-Ms M A Bigrigg and Mr M D Read set an enviable pace for long overdue improvements in the care of women with early pregnancy complications. The improvement in shortened length of stay for treated and untreated women seems inarguable, but the results for savings of bed days by avoiding or reducing admissions are derived by extrapolating from a historical control group, probably with wide confidence intervals. The financial calculations based on the extrapolations may be oversimplified. Moreover, there is no complete comparison of the resource costs of providing the service with and without an early pregnancy assessment unit. For example, there seems to be no allowance for the resource costs of running the assessment unit, for the specific costs and effects of making ultrasonography and haematology services available seven days a week, or for undetected secondary changes in practice by general practitioners and hospital doctors. The costing of bed days alone is a complex matter, and may even have been affected by the new system. A complete economic appraisal for cost effectiveness can also be complicated,2 yet this exercise was not described in detail, which clouds its otherwise important message. Although we are inspired by the initiative of Ms Bigrigg and Mr Read, we suspect that our local budget managers would require more complete analyses before rearranging the use of these resources. ANDREW J DAWSON Department of Obstetrics and Gynaecology, Unisersity of Wales College of Medicine, Cardiff CF4 4XN DAVID R COHEN Polvtechnic of Wales,
Pontylpridd, Mid Glamorgan CF37 1DL I Bigrigg MA, Read MD. Management of women referred to early pregnancy assessment unit: care and cost effectiveness. BM.7 1991;302:577-9. (9 March.) 2 Drummond MF, Stoddart GL, Torrance GW. Methods for the economtc evaluation of health care programmes. Oxford: Oxford Medical Publications, 1987.
elimination of the subsequent pill free interval if the last seven pills were implicated.' Yet I agree with Professor Orme and Dr Back that we must seek other explanations for most of the breakthrough conceptions in Ms Anne Fleissig's study.4 I suggest that most were due to errors in taking the pill, specifically those causing lengthening of the pill free interval, which we now know to be a time when there is some return of follicular activity approaching close to actual ovulation in a few women.' Unfortunately, few women are taught that being a bit late in starting the next packet is far more dangerous in terms of contraception than missing even several tablets in midpacket. And if they make a break in taking the pill near the end of a packet few are aware that they should run on to the next packet with no (superimposed) regular pill free time that month. Dr C. B Everett asks, "Do we now need to increase the strength of oral contraceptives used by young women?"6 In my opinion this would be a retrograde step. Far more logical would be to direct attention to that pill free interval. For a start, all modern pills ought to be packaged for 22 days followed by a six day gap, as, helpfully, was the case with Organon's pill Minilyn (now withdrawn). This would increase the margin for error despite a negligible increase in hormone load; and there is an extra advantage for compliance in that the starting day is the same as the finishing day for each pack. While we wait for the manufacturers to heed this oft repeated recommendation, if a woman has a breakthrough pregnancy it is even now the policy of the Margaret Pyke Centre to suggest that she "tricycles," using a monophasic pill. This means taking three or four packets in a row' followed by a (shortened) pill free interval. This simply reduces the number of pill free episodes, dangerous in terms of contraception, to four or five a year from the usual 13. This is vastly preferable to rebuking the woman, even when she admits to forgetting one or two tablets-as so many do this with impunity the ability to conceive probably indicates that that woman is regularly closer than average to ovulation at the end of each seven day pill free interval. If the pill free interval is less frequent and shorter there is a greater margin for error. JOHN GUILLEBAUD
Margaret Pvke Centre, London W I1V 5TW I Orme Mt, Back DJ. Unintended pregnancies and contraceptive use. BAMJ 1991;302:789. (30 March.) 2 Sparrow MJ. Pill method failure. NZ Med] 1987;100:1102-5. 3 Guillebaud J. Contraception-your questions answered. Edinburgh: Churchill Livingstone, 1989:74-86. 4 Fleissig A. Unintended pregnancies and the use of contraception: changes from 1984 to 1989. BMJ 1991;302:147. (19 January.) 5 Guillebaud J. The forgotten pill-and the paramount importance of the pill-free week. British Journal oJf Family Planning
1987;12(suppl):35-43.
Unintended pregnancies and contraceptive use SIR,-I know from discussions with Professor M Orme and Dr D J Back that it would be wrong so to misinterpret their recent letter' as to consider all gastrointestinal upsets and drug interactions unimportant as potential causes of failure of the contraceptive pill. Vomiting remains highly relevant-indeed, it was the commonest identifiable explanation for unplanned conception in a study of women taking the pill apparently consistently.2 Though it is perfectly true that broad spectrum antibiotics reduce efficacy in only a tiny number of women, the prescriber does not know who these are. Enzyme inducers, especially rifampicin, griseofulvin, and most anticonvulsants, significantly reduce blood concentrations of both oestrogen and progestogen. So for short term use of any interacting drug it is prudent to recommend the usual advice given in leaflets of the United Kingdom Family Planning Association. This is to take extra contraceptive precautions throughout and for seven days after the drug treatment, with
6 Everett CB. Unintended pregnancies and contraceptive use. BMJ 1991;302:789-90. (30 March.)
Phosphate enemas in childhood SIR,-We read with interest Dr M McCabe and colleagues' lesson of the week concerning phosphate enemas in childhood.' We became aware of the possible dangers of phosphate enemas whilst preparing our report on magnesium enemas.' In our original submission these dangers were mentioned, but pressure of space led to their not being included. It therefore came as a surprise to read that we had "implied that phosphate enemas might be an alternative" (to magnesium sulphate enemas). Our report questioned the justification of administering magnesium sulphate enemas to children, and it emphasised the importance of the accurate, specific prescribing of enemas and the checking of the type of enema to be given as the packaging of Fletcher's phosphate and magnesium sulphate enemas is very similar. This was the only mention of phosphate enemas-a 1273
far cry from implying that their use should be encouraged. MARK ASHTON MICK NIELSON DAVID SUTTON P'riicess Aninec Hospital,
Southampton S09 4HA \IcCabc M, Sibert JR, Rouitledge I'A. I'hosphate cncmas in childhood: a cause for concern. 1BMlI7 1991;302:1074. (4 Mma.2 2 Asltton MR. SuLtton D, Niclsen 1. Severe magnesitum toxicity aftcr magnesium sulphatc cicma in a chronically constipated chilid. 13,1,7 1990;300:541.
SIR,-In their lesson of the week on phosphate enemas in childhood' Dr M McCabe and colleagues correctly point out that the British datasheet for Fletchers' phosphate enema states: "Children: under 3, not recommended." In the past confusion may have been caused by the British National Formulary stating that the dose in children should be reduced according to age, but current editions are now consistent with the datasheet. Although absorption of phosphate has not been reported in the United Kingdom, the datasheet for the American counterpart (Fleet enema), which is more hypertonic (approximately 16% wt/vol sodium acid phosphate), records details of systemic
absorption.' Hypertonic phosphate enemas are commonly used in constipated subjects who do not respond to either dietary manipulation or oral treatment. The fact that over two million phosphate enemas are manufactured each year (Pharmax; data on file) confirms their continuing popularity. The main warning in the current British datasheet concerns the risk of inducing vasovagal episodes. We continue to emphasise the need for care in prescribing and administering phosphate enemas. Because they are almost always used in patients who are debilitated or elderly, or both, the possibility of untoward events is amplified, but such events may go unnoticed until too late. We have reviewed published reports on the safety of phosphate enemas. As well as problems caused by the constituents, injury caused by the enema nozzle occurs uncommonly but is probably underdiagnosed. In high risk patients-for example, patients with neurological impairment and those with communication difficulties -the rectum may unwittingly be perforated and chemical damage caused by the hypertonic phosphate may follow. (In one case reported in Britain rectal necrosis occurred and the patient required a defunctioning
colostomy.') With the British Data Sheet Compendium becoming available to the public,a just as the Physicians Desk Reference is on sale openly in the United States, patients may become as aware of the information therein as their doctors. Clearly, if products are used outside their strict guidelines questions may be expected. M GOLDMAN
IPharmax, Bcxle\, Kent DA5 INX I McCabe M, Sibert JR, Routlcdge PA. Phosphate enemas in childhood: cause for concern. BMI7 1991;302:1074. (4 Mai.) 2 Physicians desk reference. 45th ed. Oradell, New Jersey: Medical Economics, 1991. 3 Smith J, Carr N, Corrado OJ, Young A. Rectal necrosis after a phosphate enema. Age Ageing 1987;16:328-30. 4 Wells F. ABPI Xviewpoint. Association of Information Ojfice-rs in the Pharmameutical Indusir' Newsletter No 5 1990 October: 11-2.
Rectal examination in patients with abdominal pain SIR,-Dr Paul Kinnersley and colleagues made several comments' about our paper on rectal examination in patients with abdominal pain.2 They suggested that the sample of patients used in our study was a highly selected one. 1274
We stated in our paper that this was a consecutive series of patients seen at a district general hospital. Of the 1028 patients for whom we had data on rectal examination, 911 had been referred by their general practitioner, most of the remainder being self referred. The odds ratio, sensitivity, and specificity of rectal examination in the patients who had been referred by their general practitioner were almost identical with those in the patients who had referred themselves. This indicates that the profile of findings on rectal examination was independent of the source of the patient and suggests that the general practitioners' findings on rectal examination were not a discriminating factor for referral to hospital. Although this does suggest that rectal examination performed by general practitioners in patients with pain in the right lower quadrant might not be of value, we accept that on the basis of our data we cannot be certain that that is the case. Dr Kinnersley and colleagues also criticise the presentation of our data. They suggest that we should have given data on the specificity of signs for appendicitis. The specificity of a test conveys little useful information unless its sensitivity is also known because the diagnostic value of a test is related to the extent to which the sum of these two exceeds 100%. A test can have no diagnostic value but will have a high specificity if it occurs rarelyfor example, diagnosing only left handed patients as having appendicitis would give a specificity in the region of around 90%, but this would be offset by a very low sensitivity of around 10%. As the sum of the sensitivity and specificity does not exceed 100% this feature would clearly be of no diagnostic value. In our study the sensitivity of right sided rectal tenderness for acute appendicitis was 34% and the specificity 72%. The sum of these two figures only just exceeds 100%, indicating that right sided rectal tenderness, as we stated, is of little diagnostic value in diagnosing acute appendicitis. Comparing the figures for right sided rectal tenderness with those for rebound tenderness, which has a specificity of 62% and a sensitivity of 67%, shows that this particular test has substantial diagnostic potential. The odds ratio reflects both sensitivity and specificity and can be expressed as:
sensitivity x specificity (100 - sensitivity) x (100 - specificity) If the sum of the specificity and sensitivity is 100, this equation gives an odds ratio of 1. Clearly, odds ratios provide more useful information than is available from looking at specificity alone, particularly when the diagnostic values ofparticular signs are looked at. It is because of this that they are widely used as descriptive statistics in epidemiological studies, and they have a natural relation to analysis by multiple logistic regression. J MICHAEL DIXON R A ELTON University Department of Surgery, Royal Infirmary, Edinburgh EH3 9YW 1 Kinnersley P, Richards J, Owen P, Wilkinson C. Rectal examination in patients with abdominal pain. BMJ 1991;302: 908-9. (13 April.) 2 l)ixon JM, Elton RA, Rainey JB, Macleod DAD. Rectal examination in patients with pain in the right lower quadrant of the abdomen. BMJ, 1991;302:386-8. (16 February.)
Health education, cervical smears, and "Asian" women SIR,-Dr Brian R McAvoy and Rabia Raza's interesting study' was marred by their definition of the term "Asian"; this emphasises the need for accuracy and precision in definitions. They use the term Asian to refer to those "of New Commonwealth and Pakistani ethnic origin or descent, including those from Bangladesh and east Africa."
By their definition the study would include people of Chinese origin born in Singapore or Malaysia, and this is not what they intend. Perhaps they should have defined Asian as being those who are of Indian subcontinent origin or descent, including those from India, Pakistan, Bangladesh, and east Africa. One way of underlining the specific use of the term Asian is to put it in parentheses or italics, as some authors have done.2 HARPREET S KOHLI
Monklands and C umbernauld Unit, Lanarkshire Health Board, Coatbridge, Lanarkshire l\L5 3BN I McAvoy BR, Raza R. Can health education incrcase uptake of cerv'ical smear testing among Asian women? AM 7 1991;302:
833-6. (6 April.) 2 Kohli HS. The health of "Asians" in Britaini. Nattional Mfedical Journal of India 1988;1:27-30. 3 Firdous R, Bhopal RS. Rcproductive health of "Asiani" women: a comparative study with hospital and community perspcctives. Public Health 1999;103:307-15.
Benzodiazepines and pilot error SIR,-Ms Daphne Gloag's article on air crashes and human error highlighted the problems faced by air crews and traffic controllers.' A recent review has shown that, despite its dangers, the use of alcohol is still not uncommon among pilots.2 Time changes and jet lag may increase a pilot's urge to take a couple of drinks, and the related sleep disturbance is perhaps an understandable reason to consider the use of a hypnotic. To prevent residual sedation air crew and others whose occupations demand vigilance, motor skill, or decision making may be advised to take a short acting hypnotic such as triazolam, midazolam, or brotizolam.3 Because of triazolam's short half life of only two to three hours daytime sedation is rare, but amnesia-a recognised side effect of such short acting benzodiazepines-may persist after sedation has disappeared.4 This has become known as "traveller's amnesia. "9 As a result of using triazolam pilots may be suffering from impaired memory function without being aware of it. In other words, they may have done, or not done, things without having stored them in their memory. The potential danger of periodic amnesia in pilots has attracted little attention. In an authoritative review of the treatment of insomnia Gillin and Byerley briefly referred to the risk of traveller's amnesia but did not consider its hazard with regard to air traffic staff.6 As well as causing amnesia, drugs such as triazolam and midazolam have been associated with a variety of side effects, including increased daytime anxiety, behavioural abnormalities, and psychotic episodes.7'-" Selecting a safe hypnotic for pilots is difficult. It is not clear whether the ignition-interlock system, recently proposed as a convenient practical test for pilots,2 will be able to detect drug induced memory impairment. R H B MEYBOOM
'I'hc Netherlands Centre for Monitoring of Adverse Reactions to Drugs, PO Box 5406, 2280 HK Rijswijk, The Netherlands 1 Gloag D. Air crashes and human error. BMNJ 1991;302:550. (9 March.) 2 Model JG, Mountz JM. Drinking and flying-the problem of alcohol use by pilots. N Engli Med 1990;323:455-61. 3 Nicholson AN. Hypnotics and occupational medicine. fOccup Med 1990;32:335-41. 4 Scharf MB, Kauffman R, Brown L, Segal jj, Hirschowitz J. Morning amnestic effects of triazolam. Hillside J Clin Psychiatry 1986;8:38-45. S Morris HH, Estes 1ML. Traveler's armesia, transient global amnesia secondary to triazolam. JAMA 1987;258:945-6. 6 Gillin JC, Byerly WF. The diagnosis and management of insomnia. N Engli Med 1990;322:239-48. 7 Bixler EO, Kales A, Brubaker BH, Kales JD. Adverse reactions to benzodiazepine hypnotics: spontaneous reporting system. Phar7nacology 1987;35:286-300. 8 Tan TL, Bixler EO, Kales A, Cadieux RJ, Goodman AL.
BMJ
VOLUME 302
25 MAY 1991
Guillebaud J, eds. Contraception: science and practice. London: Butterworth. 1989:135.
SIR,-Dr Peter J M Davis expresses concern regarding the semantics used for describing postcoital contraception.' There would be more sympathy for his adoption of the "moral high ground" if his letter did not display numerous inconsistencies. There are many translations of the Hippocratic oath and the one chosen by Dr Davis strongly supports his views. He should, however, examine the other passages of this pre-Christian philosopher's thoughts on medical ethics and ask himself how many of the normal day to day activities that he accepts as part of his routine work in general practice are, in fact, proscribed by the oath. If Hippocrates's thoughts on confidentiality are accepted as sacrosanct then it would be impossible to abide by the notifiable diseases legislation; his admonitions to accept the wisdom of his teachers could prevent a practitioner ever applying to go on an updating course. Perhaps Dr Davis does follow that particular part of the oath and that is why he still believes that modern intrauterine contraceptives work predominantly by preventing implantation. (Their major effect is in preventing sperm ascent through the genital tract, thus reducing the number of fertilisations.2) I hope that Dr Davis's patients are fully informed of his views and therefore the limitations of the contraceptive "services" that he is actually prepared to provide before they sign any agreement. Hopefully the purchaser-provider philosophy will allow family health services authorities to clarify with family practitioners with such an attitude to contraception that they are offering only a limited service. Perhaps we should not be too hard on the likes of Dr Davis for displaying inconsistency. The Hippocratic texts themselves describe and advocate methods of abortion for certain pregnant females (slave prostitutes).' It was the Abortion Act 1967 that first legally enshrined this enthusiastically proclaimed right to exercise a conscience in treating patients and thus ignore another Hippocratic precept. DAVID R BROMHAM
Department of Obstetrics and (ivnaecology, St James's UniversitN Hospital, Leeds LS9 7TF I D)ais PJM. Emergency contraception. BMJ7 1991;302:1082-3. (4 May.) Sisin I. IUDs are contraceptives, not abortifacients: a comment
2
BMJ
VOLUME
302
25
MAY
1991
on research and belief. Studies in Famils Planting 1989;20: 355-9. 3 King H. Making a man: becoming human in earl5 Greek medicine. In: Dunstan GR, ed. The human embrvo: Aristotle and the,Arabic and European traditions. Exeter: University of Exeter Press, 1990: 10-9.
Early pregnancy assessment units SIR,-Ms M A Bigrigg and Mr M D Read set an enviable pace for long overdue improvements in the care of women with early pregnancy complications. The improvement in shortened length of stay for treated and untreated women seems inarguable, but the results for savings of bed days by avoiding or reducing admissions are derived by extrapolating from a historical control group, probably with wide confidence intervals. The financial calculations based on the extrapolations may be oversimplified. Moreover, there is no complete comparison of the resource costs of providing the service with and without an early pregnancy assessment unit. For example, there seems to be no allowance for the resource costs of running the assessment unit, for the specific costs and effects of making ultrasonography and haematology services available seven days a week, or for undetected secondary changes in practice by general practitioners and hospital doctors. The costing of bed days alone is a complex matter, and may even have been affected by the new system. A complete economic appraisal for cost effectiveness can also be complicated,2 yet this exercise was not described in detail, which clouds its otherwise important message. Although we are inspired by the initiative of Ms Bigrigg and Mr Read, we suspect that our local budget managers would require more complete analyses before rearranging the use of these resources. ANDREW J DAWSON Department of Obstetrics and Gynaecology, Unisersity of Wales College of Medicine, Cardiff CF4 4XN DAVID R COHEN Polvtechnic of Wales,
Pontylpridd, Mid Glamorgan CF37 1DL I Bigrigg MA, Read MD. Management of women referred to early pregnancy assessment unit: care and cost effectiveness. BM.7 1991;302:577-9. (9 March.) 2 Drummond MF, Stoddart GL, Torrance GW. Methods for the economtc evaluation of health care programmes. Oxford: Oxford Medical Publications, 1987.
elimination of the subsequent pill free interval if the last seven pills were implicated.' Yet I agree with Professor Orme and Dr Back that we must seek other explanations for most of the breakthrough conceptions in Ms Anne Fleissig's study.4 I suggest that most were due to errors in taking the pill, specifically those causing lengthening of the pill free interval, which we now know to be a time when there is some return of follicular activity approaching close to actual ovulation in a few women.' Unfortunately, few women are taught that being a bit late in starting the next packet is far more dangerous in terms of contraception than missing even several tablets in midpacket. And if they make a break in taking the pill near the end of a packet few are aware that they should run on to the next packet with no (superimposed) regular pill free time that month. Dr C. B Everett asks, "Do we now need to increase the strength of oral contraceptives used by young women?"6 In my opinion this would be a retrograde step. Far more logical would be to direct attention to that pill free interval. For a start, all modern pills ought to be packaged for 22 days followed by a six day gap, as, helpfully, was the case with Organon's pill Minilyn (now withdrawn). This would increase the margin for error despite a negligible increase in hormone load; and there is an extra advantage for compliance in that the starting day is the same as the finishing day for each pack. While we wait for the manufacturers to heed this oft repeated recommendation, if a woman has a breakthrough pregnancy it is even now the policy of the Margaret Pyke Centre to suggest that she "tricycles," using a monophasic pill. This means taking three or four packets in a row' followed by a (shortened) pill free interval. This simply reduces the number of pill free episodes, dangerous in terms of contraception, to four or five a year from the usual 13. This is vastly preferable to rebuking the woman, even when she admits to forgetting one or two tablets-as so many do this with impunity the ability to conceive probably indicates that that woman is regularly closer than average to ovulation at the end of each seven day pill free interval. If the pill free interval is less frequent and shorter there is a greater margin for error. JOHN GUILLEBAUD
Margaret Pvke Centre, London W I1V 5TW I Orme Mt, Back DJ. Unintended pregnancies and contraceptive use. BAMJ 1991;302:789. (30 March.) 2 Sparrow MJ. Pill method failure. NZ Med] 1987;100:1102-5. 3 Guillebaud J. Contraception-your questions answered. Edinburgh: Churchill Livingstone, 1989:74-86. 4 Fleissig A. Unintended pregnancies and the use of contraception: changes from 1984 to 1989. BMJ 1991;302:147. (19 January.) 5 Guillebaud J. The forgotten pill-and the paramount importance of the pill-free week. British Journal oJf Family Planning
1987;12(suppl):35-43.
Unintended pregnancies and contraceptive use SIR,-I know from discussions with Professor M Orme and Dr D J Back that it would be wrong so to misinterpret their recent letter' as to consider all gastrointestinal upsets and drug interactions unimportant as potential causes of failure of the contraceptive pill. Vomiting remains highly relevant-indeed, it was the commonest identifiable explanation for unplanned conception in a study of women taking the pill apparently consistently.2 Though it is perfectly true that broad spectrum antibiotics reduce efficacy in only a tiny number of women, the prescriber does not know who these are. Enzyme inducers, especially rifampicin, griseofulvin, and most anticonvulsants, significantly reduce blood concentrations of both oestrogen and progestogen. So for short term use of any interacting drug it is prudent to recommend the usual advice given in leaflets of the United Kingdom Family Planning Association. This is to take extra contraceptive precautions throughout and for seven days after the drug treatment, with
6 Everett CB. Unintended pregnancies and contraceptive use. BMJ 1991;302:789-90. (30 March.)
Phosphate enemas in childhood SIR,-We read with interest Dr M McCabe and colleagues' lesson of the week concerning phosphate enemas in childhood.' We became aware of the possible dangers of phosphate enemas whilst preparing our report on magnesium enemas.' In our original submission these dangers were mentioned, but pressure of space led to their not being included. It therefore came as a surprise to read that we had "implied that phosphate enemas might be an alternative" (to magnesium sulphate enemas). Our report questioned the justification of administering magnesium sulphate enemas to children, and it emphasised the importance of the accurate, specific prescribing of enemas and the checking of the type of enema to be given as the packaging of Fletcher's phosphate and magnesium sulphate enemas is very similar. This was the only mention of phosphate enemas-a 1273
far cry from implying that their use should be encouraged. MARK ASHTON MICK NIELSON DAVID SUTTON P'riicess Aninec Hospital,
Southampton S09 4HA \IcCabc M, Sibert JR, Rouitledge I'A. I'hosphate cncmas in childhood: a cause for concern. 1BMlI7 1991;302:1074. (4 Mma.2 2 Asltton MR. SuLtton D, Niclsen 1. Severe magnesitum toxicity aftcr magnesium sulphatc cicma in a chronically constipated chilid. 13,1,7 1990;300:541.
SIR,-In their lesson of the week on phosphate enemas in childhood' Dr M McCabe and colleagues correctly point out that the British datasheet for Fletchers' phosphate enema states: "Children: under 3, not recommended." In the past confusion may have been caused by the British National Formulary stating that the dose in children should be reduced according to age, but current editions are now consistent with the datasheet. Although absorption of phosphate has not been reported in the United Kingdom, the datasheet for the American counterpart (Fleet enema), which is more hypertonic (approximately 16% wt/vol sodium acid phosphate), records details of systemic
absorption.' Hypertonic phosphate enemas are commonly used in constipated subjects who do not respond to either dietary manipulation or oral treatment. The fact that over two million phosphate enemas are manufactured each year (Pharmax; data on file) confirms their continuing popularity. The main warning in the current British datasheet concerns the risk of inducing vasovagal episodes. We continue to emphasise the need for care in prescribing and administering phosphate enemas. Because they are almost always used in patients who are debilitated or elderly, or both, the possibility of untoward events is amplified, but such events may go unnoticed until too late. We have reviewed published reports on the safety of phosphate enemas. As well as problems caused by the constituents, injury caused by the enema nozzle occurs uncommonly but is probably underdiagnosed. In high risk patients-for example, patients with neurological impairment and those with communication difficulties -the rectum may unwittingly be perforated and chemical damage caused by the hypertonic phosphate may follow. (In one case reported in Britain rectal necrosis occurred and the patient required a defunctioning
colostomy.') With the British Data Sheet Compendium becoming available to the public,a just as the Physicians Desk Reference is on sale openly in the United States, patients may become as aware of the information therein as their doctors. Clearly, if products are used outside their strict guidelines questions may be expected. M GOLDMAN
IPharmax, Bcxle\, Kent DA5 INX I McCabe M, Sibert JR, Routlcdge PA. Phosphate enemas in childhood: cause for concern. BMI7 1991;302:1074. (4 Mai.) 2 Physicians desk reference. 45th ed. Oradell, New Jersey: Medical Economics, 1991. 3 Smith J, Carr N, Corrado OJ, Young A. Rectal necrosis after a phosphate enema. Age Ageing 1987;16:328-30. 4 Wells F. ABPI Xviewpoint. Association of Information Ojfice-rs in the Pharmameutical Indusir' Newsletter No 5 1990 October: 11-2.
Rectal examination in patients with abdominal pain SIR,-Dr Paul Kinnersley and colleagues made several comments' about our paper on rectal examination in patients with abdominal pain.2 They suggested that the sample of patients used in our study was a highly selected one. 1274
We stated in our paper that this was a consecutive series of patients seen at a district general hospital. Of the 1028 patients for whom we had data on rectal examination, 911 had been referred by their general practitioner, most of the remainder being self referred. The odds ratio, sensitivity, and specificity of rectal examination in the patients who had been referred by their general practitioner were almost identical with those in the patients who had referred themselves. This indicates that the profile of findings on rectal examination was independent of the source of the patient and suggests that the general practitioners' findings on rectal examination were not a discriminating factor for referral to hospital. Although this does suggest that rectal examination performed by general practitioners in patients with pain in the right lower quadrant might not be of value, we accept that on the basis of our data we cannot be certain that that is the case. Dr Kinnersley and colleagues also criticise the presentation of our data. They suggest that we should have given data on the specificity of signs for appendicitis. The specificity of a test conveys little useful information unless its sensitivity is also known because the diagnostic value of a test is related to the extent to which the sum of these two exceeds 100%. A test can have no diagnostic value but will have a high specificity if it occurs rarelyfor example, diagnosing only left handed patients as having appendicitis would give a specificity in the region of around 90%, but this would be offset by a very low sensitivity of around 10%. As the sum of the sensitivity and specificity does not exceed 100% this feature would clearly be of no diagnostic value. In our study the sensitivity of right sided rectal tenderness for acute appendicitis was 34% and the specificity 72%. The sum of these two figures only just exceeds 100%, indicating that right sided rectal tenderness, as we stated, is of little diagnostic value in diagnosing acute appendicitis. Comparing the figures for right sided rectal tenderness with those for rebound tenderness, which has a specificity of 62% and a sensitivity of 67%, shows that this particular test has substantial diagnostic potential. The odds ratio reflects both sensitivity and specificity and can be expressed as:
sensitivity x specificity (100 - sensitivity) x (100 - specificity) If the sum of the specificity and sensitivity is 100, this equation gives an odds ratio of 1. Clearly, odds ratios provide more useful information than is available from looking at specificity alone, particularly when the diagnostic values ofparticular signs are looked at. It is because of this that they are widely used as descriptive statistics in epidemiological studies, and they have a natural relation to analysis by multiple logistic regression. J MICHAEL DIXON R A ELTON University Department of Surgery, Royal Infirmary, Edinburgh EH3 9YW 1 Kinnersley P, Richards J, Owen P, Wilkinson C. Rectal examination in patients with abdominal pain. BMJ 1991;302: 908-9. (13 April.) 2 l)ixon JM, Elton RA, Rainey JB, Macleod DAD. Rectal examination in patients with pain in the right lower quadrant of the abdomen. BMJ, 1991;302:386-8. (16 February.)
Health education, cervical smears, and "Asian" women SIR,-Dr Brian R McAvoy and Rabia Raza's interesting study' was marred by their definition of the term "Asian"; this emphasises the need for accuracy and precision in definitions. They use the term Asian to refer to those "of New Commonwealth and Pakistani ethnic origin or descent, including those from Bangladesh and east Africa."
By their definition the study would include people of Chinese origin born in Singapore or Malaysia, and this is not what they intend. Perhaps they should have defined Asian as being those who are of Indian subcontinent origin or descent, including those from India, Pakistan, Bangladesh, and east Africa. One way of underlining the specific use of the term Asian is to put it in parentheses or italics, as some authors have done.2 HARPREET S KOHLI
Monklands and C umbernauld Unit, Lanarkshire Health Board, Coatbridge, Lanarkshire l\L5 3BN I McAvoy BR, Raza R. Can health education incrcase uptake of cerv'ical smear testing among Asian women? AM 7 1991;302:
833-6. (6 April.) 2 Kohli HS. The health of "Asians" in Britaini. Nattional Mfedical Journal of India 1988;1:27-30. 3 Firdous R, Bhopal RS. Rcproductive health of "Asiani" women: a comparative study with hospital and community perspcctives. Public Health 1999;103:307-15.
Benzodiazepines and pilot error SIR,-Ms Daphne Gloag's article on air crashes and human error highlighted the problems faced by air crews and traffic controllers.' A recent review has shown that, despite its dangers, the use of alcohol is still not uncommon among pilots.2 Time changes and jet lag may increase a pilot's urge to take a couple of drinks, and the related sleep disturbance is perhaps an understandable reason to consider the use of a hypnotic. To prevent residual sedation air crew and others whose occupations demand vigilance, motor skill, or decision making may be advised to take a short acting hypnotic such as triazolam, midazolam, or brotizolam.3 Because of triazolam's short half life of only two to three hours daytime sedation is rare, but amnesia-a recognised side effect of such short acting benzodiazepines-may persist after sedation has disappeared.4 This has become known as "traveller's amnesia. "9 As a result of using triazolam pilots may be suffering from impaired memory function without being aware of it. In other words, they may have done, or not done, things without having stored them in their memory. The potential danger of periodic amnesia in pilots has attracted little attention. In an authoritative review of the treatment of insomnia Gillin and Byerley briefly referred to the risk of traveller's amnesia but did not consider its hazard with regard to air traffic staff.6 As well as causing amnesia, drugs such as triazolam and midazolam have been associated with a variety of side effects, including increased daytime anxiety, behavioural abnormalities, and psychotic episodes.7'-" Selecting a safe hypnotic for pilots is difficult. It is not clear whether the ignition-interlock system, recently proposed as a convenient practical test for pilots,2 will be able to detect drug induced memory impairment. R H B MEYBOOM
'I'hc Netherlands Centre for Monitoring of Adverse Reactions to Drugs, PO Box 5406, 2280 HK Rijswijk, The Netherlands 1 Gloag D. Air crashes and human error. BMNJ 1991;302:550. (9 March.) 2 Model JG, Mountz JM. Drinking and flying-the problem of alcohol use by pilots. N Engli Med 1990;323:455-61. 3 Nicholson AN. Hypnotics and occupational medicine. fOccup Med 1990;32:335-41. 4 Scharf MB, Kauffman R, Brown L, Segal jj, Hirschowitz J. Morning amnestic effects of triazolam. Hillside J Clin Psychiatry 1986;8:38-45. S Morris HH, Estes 1ML. Traveler's armesia, transient global amnesia secondary to triazolam. JAMA 1987;258:945-6. 6 Gillin JC, Byerly WF. The diagnosis and management of insomnia. N Engli Med 1990;322:239-48. 7 Bixler EO, Kales A, Brubaker BH, Kales JD. Adverse reactions to benzodiazepine hypnotics: spontaneous reporting system. Phar7nacology 1987;35:286-300. 8 Tan TL, Bixler EO, Kales A, Cadieux RJ, Goodman AL.
BMJ
VOLUME 302
25 MAY 1991
