[Changes in the isoniazid inactivation index during antituberculosis treatment in children].

Fluorimetric assays of serum isoniazid levels were performed in children treated either with isoniazid alone, or with isoniazid associated with other antituberculous drugs, in order to adjust the dosage. Study of the inactivation index at the 3rd hour, showed a faster metabolism in the younger children. Assays repeated twice or more in 160 patients, showed important variability in the results. The variation coefficient was 18,6% for the whole group and above 25% in 25% of cases. The younger age and the association of several other drugs (corticosteroids and possibly erythromycine) may be responsible, at least partly, for this variability. However, association with antituberculous drugs (ethambutol, rifampin) does not seem to change significantly isoniazid metabolism. The magnitude of these variations should lead to a greater caution in the interpretation of isolated isoniazid dosages.