2-774), and thus couples who want to have children will continue to present. If a couple decide to have a child after they have been informed of the risks we advise the man to have a sperm count to rule out oligospermia and issue ovulation kits with advice to have unprotected intercourse only at the time of ovulation. As soon as pregnancy is confirmed the woman is referred to the consultant obstetrician in our hospital to register for antenatal care. We suggest a further HIV test at three months' gestation when termination might be an option, and we stress the need to use condoms throughout the pregnancy. The mother and baby are tested postnatally, and provided that the mother remains uninfected with HIV and condoms are used we advise that breast feeding is safe. Despite being aware of the risks couples will continue to have children. We believe that information should be provided to reduce these risks as far as possible. CHRISTINE LEE RIVA MILLER ELEANOR GOLDMAN Royal Free Hospital, London NW3 2QG 1 Norman S, Studd J, Johnson M. HIV infection in women. BMJ 1990;301:1231-2. (1 December.) 2 Haemophilia Society. Haemofact AIDS. No 7. London: Haemophilia Society, 1985. 3 Miller R, Bor R. Counselling for HIV screening in women. In: Studd J, ed. Progress in obstetrics and gynaecology. Vol 8. London: Churchill Livingstone, 1990:175-90. 4 Lee CA, Phillips A, Elford J, et al. The natural history of human immunodeficiency virus infection in a haemophiliac cohort. Br7 Haematol 1989;73:228-34.

HIV and surgeons SIR,-Messrs David Hamblen and Geoffrey Newton describe the risk of HIV infection among orthopaedic surgeons.' We think that it is time to draw attention to the burden of surgeons and other medical staff (especially midwives) working in places where a fifth or more of all patients coming to the operating room or delivery ward are positive for HIV antibodies. We have used the formula 1 -(1 - fp)f to calculate the risk of HIV infection, where f is the prevalence of HIV, p the risk of transmission, and n the number of injuries (R A Coutinho, personal communication). If the risk of infection with HIV is estimated at 0 5% after each injury (needlestick or instrumental) and the number of injuries is 10 a year the risk of infection with HIV is 26% over a 30 year career. We wonder how many surgeons in industrialised countries, given the sometimes difficult work circumstances, would accept this degree of risk. A DE CLERCQ

concentrations of the drug was thought to be responsible.2 A retrospective study has suggested an association between the septic complications of diverticular disease and non-steroidal anti-inflammatory drugs.' The drugs inhibit prostaglandin synthetase, and experimental evidence has suggested that a decrease in mucosal prostaglandins may, by abolishing a "cytoprotective" effect, compromise intestinal integrity resulting in an increased susceptibility to transmucosal migration of luminal toxins.4 During 12 months we have treated three elderly constipated patients who developed stercoraceous perforations of the sigmoid colon while taking indomethacin, naproxen, and a sustained release preparation of ketoprofen. All three cases were reported to the Committee on Safety of Medicines. Although the development of these lesions was undoubtedly multifactorial and constipation the prime factor, the suspicion of an association with non-steroidal anti-inflammatory drugs seems justified. Impairment of mucosal resistance to the trauma induced by faecal stasis in patients taking these drugs could be a contributory factor. Another study has shown that 26% of patients admitted with small and large bowel perforations and haemorrhage were taking non-steroidal antiinflammatory drugs.' There are fewer than 50 reported cases of stercoraceous perforation, and in those in which drugs are implicated it is because of their constipating side effects.67 Surprisingly, non-steroidal anti-inflammatory drugs are rarely mentioned, and a causal association has never been considered. This may be explained by a failure to document that patients were taking these drugs, particularly if an aetiological role is not considered. Although the potential upper gastrointestinal effects of non-steroidal anti-inflammatory drugs have been widely emphasised, more progress is needed to define the incidence of and those patients at greatest risk from adverse lower gastrointestinal reactions. J HOLLINGWORTH

Selly Oak Hospital, Birmingham B29 6JD 1 Clements D, Williams GT, Rhodes J. Colitis associated with

ibuprofen. BMJ 1990;301:987. (27 October.) 2 Day TK. Intestinal perforation associated with osmotic slow-

release indomethacin capsules. BMJ 1983;287:167 1. 3 Corder A. Steroids, non-steroidal anti-inflammatory drugs, and serious septic complications of diverticular disease. BMJ 1987;295: 1238. 4 Robert A. An intestinal disease produced experimentally by prostaglandin deficiency. Gastroenterology 1975;69: 1045-7. 5 Langman MJS, Morgan L, Worral A. Use of anti-inflammatory drugs by patients admitted with small or large bowel perforation and haemorrhage. BMJ 1985;290:347-9. 6 Stringer MD, Greenfield S, McIrvine AJ. Stercoral perforation of the colon following postoperative analgesia. J7 R Soc Med 1987;80: 115-6. 7 Cass AJ. Stercoral perforation: case of drug induced impaction.

BMJ 1978;ii:932-3.

L OEYEN

Analysing ordered categorical data

BP 934 Kigali, Rwanda 1 Hamblen D, Newton G. HIV and surgeons. BMJf 1990;301: 1216-7. (24 November.)

Colitis associated with ibuprofen SIR,-The report by Dr D Clements and colleagues of colitis associated with ibuprofen' is further evidence that non-steroidal anti-inflammatory drugs may cause ileocolonic damage. The role of these drugs in colonic disease remains poorly defined and may be underestimated, reports being uncommon, often anecdotal, and difficult to evaluate. These drugs can cause conditions other than colitis. The osmotic sustained release preparation of indomethacin, Osmosin, was withdrawn after reports of ileocolonic perforation. Exposure of the lower bowel to particularly high BMJ

VOLUME 302

5 JANUARY 1991

SIR,-Dr Anthony P Morton and Professor Annette J Dobson write that when ordered data are treated as if they were continuous the differences between the means and medians of the two samples are meaningless point estimates.' They state that using ct and b obviates this problem but do not mention that these statistics pose severe interpretational problems of their own. Consider the results from two trials, each with 16 patients and two treatments, A and B. The patients are measured on a 10 point scale. The results from the two trials have been sorted within groups and are as follows: TrialI A:44445555 B:5 5556666

TrialII A:12345678 B: 2 3456789

The difference between the means of the two treatment groups in each case is 1, as is the difference between the medians. Thus if the data are treated as continuous the results from both trials are consistent with the theory that the effect of B compared with A is to improve results by one category. Of course the confidence limits, by virtue of the difference in variability, will be different. If 6 is calculated for the two trials, however, we get a value of 0 75 for trial I and of 0 234 for trial II. Thus b reflects scale as well as location. As patients in clinical trials cannot and should not be regarded as a random sample from a target population2 reflecting scale is an undesirable property of point estimators. It is thus incorrect to say that using i and 6 obviates problems with ordered categorical data. STEPHEN SENN Ciba-Geigy, 4002 Basle, Switzerland 1 Morton A, Dobson A. Analysing ordered categorical data from two independent samples. BMJ 1990;301:971-3. (27 October.) 2 Senn S, Auclair P. The graphical representation of clinical trial with particular reference to measurements over time. Stat Med 1990;9: 1287-302.

Incidence of advanced renal failure SIR,-Professor Mary G McGeown and Dr T G Feest and colleagues provided important new information on the incidence and prevalence of advanced chronic renal failure that highlights the underreferral of patients in the United Kingdom. "I Their estimate of 78 new patients per million population per year, however, may well be an underestimate of the incidence in some health districts. We recently completed a review of the renal services for the South East Thames Regional Health Authority. One of the main points considered in the review was the level of need across the region and how best the services could develop to cope with increasing demand for treatment. The region represents a wide range of demographic and social groups. Important characteristics determining variation in treatment rate among districts included the age structure, distance of residence from a renal unit, and the proportion of AfroCaribbeans. In the United States black people have been shown to have a higher incidence and prevalence of chronic renal failure than white people.3 Dr Feest and colleagues could not study AfroCaribbeans as their study population did not include a wide range of ethnic groups. They did, however, report that Asians did not seem to have a significantly different incidence of chronic renal failure from white people. In South East Thames region the annual acceptance rate of new patients in 1988-9, excluding nonresidents, was 49-3 per million, slightly below the United Kingdom average of 55-1 estimated by the European Dialysis and Transplant Association.4 The treatment rate varied considerably among districts, however, with the acceptance rate of new patients in Camberwell and west Lambeth (123-5 and 83 per million per year respectively) already being well above the incidence of 78 new patients per million estimated by Dr Feest and colleagues. The high treatment rate of new patients in these districts may be partly explained by the presence of renal units within the districts but also by districts' high proportions of Afro-Caribbeans. The 1981 census data showed that about 22% of the populations in Camberwell and west Lambeth belonged to ethnic minorities, mainly Afro-Caribbeans. This group is known to be predisposed to hypertension, non-insulin dependent diabetes mellitus, and systemic lupus erythematosus, which in turn can lead to end stage renal failure.' In addition, there are other groups-, such as the Vietnamese, who are 51

Analysing ordered categorical data.

2-774), and thus couples who want to have children will continue to present. If a couple decide to have a child after they have been informed of the r...
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