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Accuracy and precision of two methods of blood lead estimation.

In recent years, increasing reliance has been placed upon the blood lead as a measurement ofoccupational exposure. The aim is to limit absorption before biochemical mechanisms are interfered with significantly. In the present state of knowledge, there is general agreement that a blood lead below 80,ug/100 ml rarely tesults in harmful effects, but the aim should be to strive for lower quantities than this to ensure that, ultimately, the lead worker is at no greater hazard than the ordinary citizen. This increasing use of the blood lead has been made possible by the development of micro techniques using atomic absorption spectrophotometry. There is ample evidence (see, for example, White, 1968; Delves, 1970; and CerDik and Sayers, 1971) to show that these new methods can be as precise as established analytical procedures. The coefficient of variation is of the order of 5% in the single estimate. The standard deviation can, however, be reduced by reporting on the mean of several estimates. With most macro techniques, this is too time-consuming and expensive, but using inicro methods, replication is comparatively simple. The variance of the difference between results obtained by two different methods is the sum of their individual variances. A 95% confidence interval would suggest that the difference between two individual results should lie within ± 14% (2 standard deviations). In order to demonstrate the differences that might be observed, the results of a recent investigation into the accuracy of the two routine methods of blood lead estimation used by this laboratory, viz., cathode ray polarography and atomic absorption spectrophotometry, are summarized in the Figure. Over the course of the last few months, the blood lead has been routinely estimated from venous blood samples by the atomic absorptiorn spectrophotometry micromethod (Cernik and Sayers, 1971) using a 6 5 mm punched disc. The blood lead is reported as the mean of two estimates (i.e., two discs are analysed). As a routine check, every twentieth sample is also analysed by polarography (Cernik, 1967). For the purpose of this exercise, however, these samples were also duplicated so that the variances of the two methods