Pitfalls in the American Monitor kit methods for determination of total and "direct" bilirubin.

We evaluated the American Monitor Corporation kit for total and direct-reacting bilirubin and found that it suffers serious deficiencies, which lead to inaccurate and imprecise results. The main problem with the total-bilirubin procedure is that the short reaction time (2 min) is inadequate for completion of the reaction. The poor precision of the direct-bilirubin method is due to the short reaction time and the inability of the "stabilizer" (hydroxylamine sulfate) to completely destroy the diazo reagent. Depending on when Fehling's reagent is added, the reaction time may vary from 2 min to 7 min. Values for direct bilirubin at 7 min exceed those obtained at 2 min by 17 to 29%. The short reaction time makes color development temperature dependent, an additional source of imprecision. The suboptimal concentration of the diazo reagent results in underestimation of direct-reacting bilirubin. We recommend changes that improve both precision and accuracy of the kit procedures.