arrival of the doctor, and the administration of the thrombolytic agent (either at home or in hospital), and I believe that it might well be possible to show important correlations. Drs Radford and Richards, and Dr M S Stead, state, as if it was fact, that anistreplase produces an excess of four in 1000 strokes compared with streptokinase. Is it not remarkable that the major study from which this figure derives, for which the analysis is still incomplete, has been announced to the world and yet not published, and that our information is being received at second hand through the medium of a medical journalist, however distinguished and amusing'? Without being able to see the original data relating to analysis of the whole of the study it is premature to discuss these findings. I have spoken on the telephone to Dr Rory Collins, the main organiser of the third international study of infarct survival (ISIS-3), and he has made it clear-as also become apparent in Dr Michael O'Donnell's report'-that there is no difference in survival at the end of one month between patients who are given streptokinase, tissue plasminogen activator, or anistreplase. As most of the patients who suffered stroke died the disadvantage of the stroke has already been largely discounted four weeks later by a compensatory reduction in mortality from other causes, presumably cardiac causes, among patients given anistreplase. These are curious observations, and they cannot be interpreted without much more information about the patients who suffered the strokes and their particular characteristics. Dr Stead hopes that nobody will be persuaded to give anistreplase to him should he be unfortunate enough to suffer a myocardial infarction, and his belief that he would be at risk of a four in 1000 increase in stroke is emotionally understandable but needs to be put into perspective. On the basis of the present tentative, interim, and unpublished data, if Dr Stead was administering anistreplase rather than receiving it he might encounter one patient in whom his treatment was associated with a stroke once or possiblv twice in 500 years. CLIFFORD R KAY

The hope expressed by Drs Russell Jones and White that a method of assessing the protection afforded by sunscreens against ultraviolet A radiation could be agreed at the meeting of the Commission Internationale de l'Eclairage, which met just before the international conference, has not been realised. Again, we attended this meeting, and from the vigorous debate on the merits and otherwise of various in vivo and in vitro methods2 it is clear that further discussions will be needed before a single method can be agreed by manufacturers and governmental regulators. It should be remembered that when sunscreens are applied for cosmetic purposes the user is not exposed to either ultraviolet B or ultraviolet A radiation but to solar ultraviolet radiation, which always contains both. For this reason the terms ultraviolet B sun protection factor and ultraviolet A sun protection factor have little meaning. As a consequence we suggest that to have a numerical statement specifically of protection against ultraviolet A radiation beside the sun protection factor on product packs would be unnecessary and potentially confusing. If manufacturers feel compelled to provide information on the broadness of protection against different ultraviolet wavelengths we believe that this can be obtained adequately from the ratio of spectral absorbances in different ultraviolet wavebands measured by in vitro assay. This approach obviates the need for further human studies using clinical end points, some of which are of dubious value in relation to protection from exposure to the sun. The sun protection factor should remain the most important measure of protection against the harmful effects of solar ultraviolet radiation and continue to be determined by phototesting human subjects. B L DIFFEY

Regional Medical l'hvsics Department, Dryburn Hospital, Durham DH I 5TW

N J LOWE Skin Research Foundation of California and Division of Dermatology, University of California at Los Angeles School of Medicine, Los Angeles,

Manchester Research Unit, Royal College of General Practitioners, Manchester M20 OTR

California 90024, United States

I Radford J, Richards RG. Domiciliary thrombolytic treatment. BMJ 1991;303:120. (13 July.) 2 Stead MS. Domiciliary thrombolytic treatment. BMJ 1991;303: 120. (13 July.) 3 O'Donnell M. Battle of the clotbusters. BMJ 1991;302:125961. (25 May.)

1 Russell Jones R, White IR. Assessing protective effect of sunscreen products. BMJ 1991;303:52. (6 July.) 2 Lowe NJ. UVA photoprotection. In: Lowe NJ, Shaath NA, eds. Sunscreens-development, evaluation and regulatorv aspects. New York: Marcel Dekker, 1990:459-68. 3 Diffey BL, Robson J. A new substrate to measure sunscreen protection factors throughout the ultraviolet spectrum. journal ofthe Soctety ofCosmetic Chemists 1989;40:127-33.

Assessing protective effect of sunscreen products SIR,-Drs Robin Russell Jones and Ian R White advocate that sunscreens should be labelled with two sun protection factors: one for ultraviolet B and one for ultraviolet A radiation.' They suggest that consumers could be told that sun protection factor/B stands for burning and sun protection factor/A stands for aging. We are concerned that this oversimplistic approach has little basis in fact and would confuse the public about the claims of sunscreens regarding protection against exposure to the sun. We participated in the second international conference on the biological effects of ultraviolet A radiation in San Antonio last June. The papers presented and the ensuing discussion showed that there are conflicting data on the relevant importance of different wavelengths of ultraviolet radiation in causing biochemical, histological, and clinical changes characteristic of photo-aging in animal models. Much more work needs to be done before we can even begin to extrapolate confidently from animal to human skin. To imply that ultraviolet A radiation is the major cause of photo-aging is ill advised at present.

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SIR,-Drs Robin Russell Jones and Ian R White discuss the problems associated with assessing the protective effects of sunscreen products.t The availability of sunscreens that are highly protective against ultraviolet B radiation has resulted in sunbathers receiving very high doses of ultraviolet A radiation, leading to many dermatological problems.2 To determine a product's sun protection factor the accepted "gold standard" method necessitates phototesting of human subjects. This method is not suitable for assessing the protection afforded by a product against ultraviolet A radiation (320-400 nm). A new, preferably in vitro, method of assessing a sunscreen's effectiveness that does not rely on recording erythema is required. Diffey and Robson have described such a method.3 In this, the spectral transmission of ultraviolet radiation (290-400 nm) is measured through a sample of Transpore tape with and without sunscreen applied. The tape was chosen as a readily available membrane that transmits both ultraviolet A and ultraviolet B radiation and has an irregular surface on which to apply the sunscreen in a manner similar to that used on human skin in vivo. This test cannot be used for products that are based on alcohol or oil. It has been suggested that human epidermis would

be a better choice of substrate on to which to apply the sunscreen.' We believe that it is essential to use human skin in any in vitro test and that the skin must be readily obtainable in a non-invasive manner. We have prepared samples of human stratum corneum taken on to double ground quartz slides by using the skin surface biopsy method.' This "sandwich" of quartz, glue, and skin readily transmits ultraviolet radiation throughout the range 290-400 nm; is an ideal substrate on to which to apply a sunscreen for in vitro testing of the protection factor; and can be used for testing any type of suncreen, including alcohol based and oil based products. To validate our method we calculated the in vitro protection factor of five sunscreens by using the method described by Diffey and Robson' but using skin surface biopsy specimens. We also calculated the in vivo sun protection factor of three of the products by a recognised method for humans. The table gives our results. Sun protection factors offive sunscreens tested in vivo and in vitro Sun protection factor Product

Base

In vivo

In vitro

A B C D E

Cream Lotion Cream Alcohol Alcohol

7 17 4 4* 15*

7 16 3 4 9

*Manufacturer's stated value.

We believe that an in vitro testing method similar to that described by Diffey and Robson' has two important uses. The data generated can be used to prescreen potential sunscreens for the degree to which they block ultraviolet B radiation in a manner similar to the "real" situation; the test is much cheaper and quicker than the test used in humans. More importantly, it is the only practical method available to assess the ability of a sunscreen to protect skin from ultraviolet A radiation. We hope that adding human stratum corneum to Diffey and Robson's method may speed the process of persuading the photobiological. community to accept and adopt this in vitro method for assessing potential sunscreens for their ability to block ultraviolet A radiation. ANTHONY D PEARSE CHRISTOPHER EDWARDS

lDepartment of Dermatology, University of Wales College of Medicine, Cardiff CF4 4XN I Russell Jones R, White IR. Assessing protective effect of sunscreen products. BMJ 1991;303:52. (6 July.) 2 Hawk JLM. Ultraviolet A radiation: staying within the pale. BMJ 1991;302:1036-7. 3 Diffey BL, Robson J. A new substrate to measure sunscreen protection factors throughout the ultraviolet spectrum. Journal of the Societv of Cosmetic Chemists 1989;40: 127-33. 4 Protecting man from UV exposure [Editorial]. Lancet 1991;337: 1258-9. 5 Marks R, Dawber RPMi. Skin surface biopsy: an improved technique for the examination of the horny layer. BrJ Dermatol

1971;84:117-23.

Normal antenatal management SIR,-In his article on normal antenatal management Professor Geoffrey Chamberlain stated that venous blood should be checked for ABO and rhesus groups and, if relevant, rhesus antibodies.' It should be standard practice to perform an antibody screen in all pregnant women.2 Professor Chamberlain seems to have completely discounted the fact that, besides anti-D, other antibodies in the rhesus and other systems, including the Kell, Duffy, Kidd, MNS, and P systems, have been implicated in haemolytic disease of the newborn. Hardy and Napier found approximately 0 2% of

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their prenatal population who were positive for D antigen to have unexpected, clinically important antibodies.' To fail to screen all pregnant women for unexpected antibodies is unacceptable. RONA PEPPER BRENTON WYLIE New South Wales Red Cross Blood Transfusion Service, Sydney, New South Wales 2000, Australia I Chamberlain G. Normal antenatal management. BMJ 1991;302:

774-87. (30 March.) 2 Americati Association of Blood Banks. Technical manual. 10th ed. Arlington, VA: American Association of Blood Banks, 1990:883. 3 Hardy J, Napier JAF. Red cell antibodies detected in antenatal tests on rhesus positive women in south and mid Wales, 1948-1978. Brj Obstet Gvnaecol 1981;88:91-100.

sacks normally used for refuse that has not been contaminated with sharps, endangering porters and others who help with the disposal process. Injuries acquired in this way have been reported at this hospital. Three further points therefore merit debate: firstly, sharps containers are relatively small (and costly) and should be used only for small sharp objects. Secondly, syringes and other soft contaminated refuse, should go into cheap and capacious plastic sacks or bin liners. Finally, a needle can be removed from a syringe in perfect safety, especially if fingers rather than forceps are used. I believe that these suggestions would improve safety; they would also save money. E A FRENCH Dcpartment of Hacmatology,

Univcrsity Hospital, Nottingham NG7 2UH

AUTHOR'S REPLY,-When a clinical account of a big subject is written, sometimes the enforced condensation of material leads those working on particular subjects to think that they have been forgotten. They have not, but space precludes their mention. Percentage reaction rate to blood transfusions measured by immunisation after a first and reaction to a second transfusion (derivedfrom Giblett') Blood group system

First transfusion Second traoisfusion

Rhesus Kell

Duffy Kidd MNS

6-988 0410 0-052 0-013 0-010

5 814

0-037 0-034 0010 0-005

At most antenatal clinics we test for all the private antibodies, as well as ABO and rhesus ones; the idea that I discount rhesus antibodies other than anti-D must spring from a misreading of my article as I purposely described the testing of all rhesus antibodies. Of the other antibodies in Drs Pepper and Wylie's list, Kell antibodies occasionally cause a clinical problem, but the others impinge on the management of pregnant women in diminishing amounts (table). GEOFFREY CHAMBERLAIN

Department of Obstetrics and (Gynaecology, St George's Hospital Medical School, London SW 17 ORE I Giblett ER. Erythocyte antigens and antibodies. In: Williams WJ, Beatler E, Ersler AJ, Lichtman MA, eds. Hematology. New York: McGraw-Hill, 1983.

Preventing needlestick injuries SIR,-Professor D C Anderson and colleagues are right to discuss the problem of preventing needlestick injuries.' It really was high time that the assumptions underlying some of the recommendations in current guidelines were reexamined. I suggest that another recommendation, which appears in the BMA's booklet2 and elsewhere, merits similar questioning. This is the advice that needles and syringes should not be separated after use but disposed of as a single unit. Firstly, if an unsheathed needle is a potential hazard for those who may come into contact with it later, as Professor Anderson and colleagues point out, how much more dangerous is a needle rigidly mounted on the end of a syringe? Secondly, sharps containers should not be overfilled. I doubt whether anyone has ever seen an overfilled sharps box containing nothing but needles. I suspect, however, that many people have seen sharps boxes overfilled by being used for syringe-needle combinations. Regrettably, when sharps boxes are overfilled in this way needlesyringe combinations find their way into the plastic

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I Aniderson L)C, Blower AL, Ganguli LA, PackerJMV. Preventing

needlestick inijuries. BMJ 1991;302:769-70. (30 Miarch.) 2 BMA. A code of practice for the safe use and disposal of sharps. London: BMA, 1990.

SIR,-The correspondence stimulated by our article on preventing needlestick injuries' shows that the issue of resheathing is not one that can be resolved by arbitrary decree. As Jagger et al stated, "recapping is hazardous, but the relevant question is whether recapping is more hazardous than handling exposed needles."2 Many people are injured and killed on pedestrian crossings, where the relevant danger is concentrated; yet no one would suggest that these crossings should be avoided. Some specific points made by correspondents demand a response. Dr Christopher Valentine and Ms Philippa Bright (4 May) seem to have missed a critical point.' For venepuncture, the main hazardous procedure under discussion, it is virtually always necessary to remove the needle before the blood is transferred to sample containers; the process of removing and disposing of the needle must surely be safer once it is resheathed. Incidentally, this need to remove the needle after venepuncture before transferring the sample is ignored in the BMA's poster on the subject, which has been widely circulated recently. Evidently most people prefer not to touch a contaminated exposed needle directly. We agree with Dr Paul N Goldwater (29 June) that workers in the Medical Research Council's HIV Clinical Trials Centre, especially, should heed the results of his clinical trial, which clearly pointed to the value of a resheathing device.' Mr Robin P Choudhury and Ms Susan J Cleator (29 June) rightly point out' a flaw in our quotation of Jagger et al's work, in which rates of resheathing are not known2 4; as most studies point to rates in excess of 60%, however, our crude calculation of protection versus injury from resheathing is probably an underestimate. We were interested to note that in Mr Choudhury and Ms Cleator's study of medical students-who are among the most technically incompetent at venepuncture-injury rates were the same in those who did and did not resheath needles. Dr T C Aw and colleagues (1 June) made much of some problems encountered by only the most incompetent operator'; anyone without a steady hand and eye should surely not be attempting venepuncture in the first place. A "two handed scoop" technique or holding the Saf-T-Cap while using it for resheathing is unnecessary and bad practice. Dr Malcolm Gatley and Ms Margaret Worsley (11 May) also make much of technical competence in an emergency-this is why we advocated providing resheathing devices in emergency areas.' They concede that the practices advocated are desirable in theory; surely this is a good place to start, especially where theory and human behaviour are concordant. We do not understand their reference to needles falling off-

perhaps they mean sheaths, though in our experience this is very uncommon: most can be wedged home safely while held at the hilt, and quality control of sheaths and needles is generally good. Mr David R Morgan of the BMA (11 May) quotes four organisations (the World Health Organisation, Centers for Disease Control, Department of Health, and BMA) as all agreeing that "the risks from resheathing are too great"'; in fact, three seem simply to have copied the fourth (the Centers for Disease Control). We were interested that in his survey nine of 24 occupational health doctors resheathed needles-that is, 38% of those doctors least likely to ignore official guidance. It is a particular worry that the WHO advocates not resheathing, as in much of the Third World (and some of the Second and First Worlds) containers that meet British Standard 7320 are unavailable or unaffordable, or both. Official bodies should take this into account and at least concede that where such expensive containers are not available a safe resheathing technique should be adopted. By the same token, the financial and other arguments put forward by Dr E A French (this issue) for separating the (resheathed) needle from the syringe and disposing of the syringe in a contaminated waste bag should be given some weight. We believe that these and related issues will not simply go away through the arbitrary use of directives; we hope that someone will organise a comparative trial in two similar NHS districts, one of which introduces an official policy of safe resheathing and the other a policy of safe nonresheathing; accurate data on resheathing and on injury rates at all stages should be collected prospectively. The issue should be resolved by science rather than stridor. D C ANDERSON

Department of Medicine L GANGULI

Department of Medical Microbiology, University of Manchester, Hope Hospital, Salford M6 8HD

JPACKER Department of Public Health Medicine, Salford Health Authority, Eccles M30 ONJ I Anderson DC, Blower AL, Packer JMV, Ganguli LA. Preventing needlestick injuries. B.MJ 1991;302:769-70. (30 March.) 2 Jagger J, Hunt EH, Brand-Elnaggar J, P'earson RD. Rates of needlestick injuryr caused by various devices in a university hospital. N EnglJ7 Med 1988;319:284-8. 3 Correspondence. Preventing needlestick injuries. B3MJ 1991; 302:1079 (4 May), 1147 (11 May), 1336-7 (1 June), 1602-3 (29 June);303:419,(17 August). 4 Jagger J, Hunt EH, Pearson JD. Recapping used needles: is it worse than the alternative?)J Infect Dis 1990;162:784-5.

Surgeons who undertake surgery for colorectal cancer SIR,-Messrs C S McArdle and D Hole presented data on postoperative mortality of patients with colorectal cancer. They concluded that there were "significant variations in patient outcome among surgeons" and that such "differences compromised survival."' We are not convinced that their data and statistical analysis provide sufficient evidence for their conclusion for several reasons. Firstly, no overall assessment was made of the significance of the differences among surgeons in their patients' outcome. The authors' table I shows that postoperative mortality ranged from 8% to 30% (important enough for the authors to mention in their summary). However, a x2 test for heterogeneity2 does not give a significant result (x2= 14: df= 12: p>025). These rates are unadjusted, but we doubt whether adjusting them would greatly reduce their variation: it did not do so for the hazard rate ratios in the authors' table VII. Secondly, no allowance seems to have been made for the lack of independence and multiple significance testing inherent in the 52 comparisons

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Normal antenatal management.

arrival of the doctor, and the administration of the thrombolytic agent (either at home or in hospital), and I believe that it might well be possible...
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