the process of clinical decision making rather than in the attribution of epidemiological risk. The key question is whether we gain important additional diagnostic information by estimating high density lipoprotein cholesterol and triglyceride concentrations. The 6-5 mmol/l cut off point mentioned is taken from the recommendations of the British Hyperlipidaemia Association and the European Atherosclerosis Society Study Group. We are not challenging these recommendations, but we did show the effect of different clinical strategies in implementing them. The reason for focusing on 6 5 mmol/l was that it is the concentration at which medical intervention is suggested. Our contention, which we believe is well supported by our data, is that many patients may be subjected to inappropriate advice or treatment, or both, if doctors intervene on the basis of total cholesterol concentration alone. Fasting triglyceride concentration needs to be measured to decide which treatment options to use as well as to assess prognosis. Drs Florkowski and Cramb suggest that the current recommendations for a 6-5 mmol/l threshold are mistaken and that to adequately characterise risk it may be necessary to screen for apolipoprotein A. But the objective of screening is to avoid the necessity of a detailed individual assessment in everybody. The authors of the various guidelines are presumably aware of the progressive relation between cholesterol concentration and coronary heart disease at values below 6 5 mmol/l, but believe that this is the most appropriate threshold for striking a balance between a population and individual approach to prevention. Screening must take account of the medical, social, and economic costs of false positive and false negative results and of the effectiveness of available medical interventions. The fact that apolipoprotein A may prove to be a good indicator of risk does not make it a good screening test. At present the most important task is to address the practical problems arising from the implementation of the current guidelines in clinical practice. DAVID MANT ANDREW NEILL

Radcliffe Infirmary, Oxford OX2 6HE JIM MANN University of Otago, Dunedin, New Zealand

SIR, -Dr H A W Neil and colleagues advocate that high density lipoprotein (HDL) cholesterol and triglyceride concentrations be measured only in those patients with a total cholesterol concentration >6 5 mmol/l.I This is based on the observation that only 2% of those with a cholesterol concentration 1*7 mmol/l. This follows from a surprisingly low proportion (2 2%) of subjects having a low HDL cholesterol concentration (