Determination of acetylator phenotype based on the ratio of acetylisoniazid to isoniazid in urine following an oral dose of ordinary isoniazid.

A simple method for classifying subjects as slow or rapid inactivators of isoniazid has been described. A uniform dose of 300 mg. of ordinary isoniazid was administered orally to 150 patients with pulmonary tuberculosis. The ratio of acetylisoniazid to isoniazid was determined in urine collected at hourly intervals from 4 to 8 h. At each hour the distribution of the ratios was clearly bimodal. The test based on the 5-6 h. urine collection is recommended for its convenience and excellent discrimination between slow and rapid inactivators. The agreement between this method and a ‘standard method was of the order of 97 per cent.