A Field Test for the Assay of Human Whole-blood Cholinesterase
D. R. Davies, J. D. Nicholls · BMJ · 1955
The level of cholinesterase (ChE) in the blood is a valuable index of the amount of anticholinesterase which has been absorbed into the system, and it has in fact been used for this purpose (Metcalf, 1951 Davies, 1952a; Marchand, 1952).Since anticholin- esterases are being used extensively in agriculture and for insect vector control-very often far removed from laboratory facilities-the determination of such an index under field conditions would be useful.The essential requirement of a field test is simplicity.The procedure and equipment required should be simple, few reagents be necessary, and the time taken for the performance of the test be reduced to a minimum.Finally, the test should be sufficiently uncomplicated to be carried out by non-specialist personnel.Aldridge and Davies (1952) have discussed the possi- bility of applying the generally used techniques for ChE assay to field conditions, and-came to the con- clusion that, for reasons given, these were unsuitable.They did, however, suggest that the electrometric method of Michel (1949) was satisfactory under certain circumstances.They based their suggestion on the fact that this technique lent itself to the rapid and easy estimation of considerable numbers of samples.While subsequent experience has shown this to be true, there are two major disadvantages.The pH meter is not reliable enough to use in the field; and the glass elec- trode is most certainly too fragile for this purpose.Fleisher and Pope (1954) have applied the Hestrin (1949) colorimetric method to the estimation of whole- blood ChE.Their method possesses the advantage that it is a colorimetric technique and that single estimations can be completed, including the collection of the blood sample, in twenty-five minutes.Its disadvantages as a field test are (1) relatively few tests can be performed in a given time, and (2) at least six reagents and an equally large number of manipulative processes are involved.Furthermore, the technique requires the use of a spectrophotometer.Limperos and Ranta (1953) have described a test which they have used under field conditions and which approximates to the requirements.The principle is the same as that of the electrometric method.The differ- ences are (1) that changes in pH are followed by changes in a bromthymol blue (B.T.B.) solution incorporated in the reaction mixture, and (2i the test is carried out in an unbuffered medium.The levels of enzyme activity are assessed by the colour of the solution after a given interval of twenty minutes, and these colours are green, olive green, olive brown, and orange.In practice the distinction between the first two and the second two is difficult.This is a serious disadvantage, since (as is shown below) this is the important range.A second major disadvantage is that under certain conditions of illumination the combination of blood and B.T.B.