Testing for isoniazid. An evaluation of the Arkansas method.

To better treat and eliminate tuberculosis, patient compliance must be improved. Compliance can be evaluated by measuring a drug or its metabolite in the urine. In Arkansas, a simple colorimetric method of checking the urine for isoniazid (the Potts-Cozart test) has been used for many years, but it is relatively unknown outside that state and its reliability has not been confirmed. To evaluate this test, urine was blindly tested from patients from a tuberculosis clinic. Controls included urine from patients from a substance abuse clinic and Veterans Medical Center. In more than 200 urine samples tested, no false-positives were found. Urinalysis showed normal values for three patients who were supposed to be receiving antituberculosis medication, but it is likely that these patients were noncompliant. A peculiarity of the test was that the color change with positive tests varied. To investigate this variation, absorption spectroscopy of many substances was performed. Nicotine accounted for the different shade of blue associated with the positive test, but the color produced and the absorption spectroscopy were different from isoniazid, so it did not confuse the interpretation of the results. This test for isoniazid in the urine is simple, quick, inexpensive, easy to interpret, and reliable. It also can be used to detect nicotine and its metabolites.