A method to prevent the loss of isoniazid and acetylisoniazid in human plasma.

Plasma concentrations of isoniazid and N-acetylisoniazid were measured at varying times after addition of both compounds to plasma (5 micrograms/ml) and storage at several different temperatures (-70 degrees C, -20 degrees C, 4 degrees C and 20 degrees C). The concentration of both compounds declined log-linearly with time and the half-life of decline was shorter with increasing temperature of storage. Both compounds were stable for at least 5 weeks, however, when stored at -70 degrees C. We conclude that this temperature should be used to prevent loss of both compounds during storage and transport of samples.