Objective: Many practitioners use plasma levels to determine the optimum do
sage of clozapine. The aim of this study was to determine the intra- and in
terlaboratory accuracy in assaying samples of clozapine dissolved in human
plasma.
Method: Three samples were sent to one laboratory to obtain an initial dete
rmination of accuracy (phase I). Then samples of clozapine dissolved in hum
an plasma were prepared at concentrations of 140, 310 and 580 ng/mL and des
patched on dry ice to 10 assaying centres in Australia and New Zealand. The
results of the survey were analysed and posted to each centre (phase II).
The programme was repeated using concentrations of 160, 380 and 640 ng/mL (
phase III). Samples prepared in purified water and freeze-dried samples wer
e also despatched.
Results: In phase II there were two centres with results significantly diff
erent from the mean. In phase III all the centres returned concordant resul
ts. There was a high level of consistency in the measurement of samples wit
h a maximum coefficient of variation of 0.16. The concentrations determined
by the centres, however, were significantly lower than the nominal concent
rations of the prepared solutions.
Conclusions: Clinicians in Australia and New Zealand who wish to know their
patients' plasma-clozapine levels can be confident that the result of the
assay is unlikely to vary with the choice of centre or the operator who per
forms the assay.