I describe a semi-micro method for use with a discrete analyzer, the Abbott ABA-100, requiring 75 microliter of sample for determination of both the total and direct bilirubins. This method utilizes a serum blank. It is simple and reasonably rapid. The azobilirubin formed by both the total and the direct bilirubins produces a reddish-violet color, which is measured at 550 nm. Values obtained are comparable to those by the automated Jendrassik and Grof procedure adapted by Gambino and Schreiber (total r = 0.999, direct r = 0.990). The method is linear to 300 mg/liter. Day-to-day precision (CV) for the total bilirubin was 8.8% for a 8 mg/liter sample (n = 45) and 4.7% for a 57 mg/liter sample (n = 46). The absorbance of hemoglobin and its derivatives (540 nm) caused a false decrease in the measured bilirubin content, the direct being more affected than the total. This problem was alleviated by the use of caffeine and acid blanks.
A sensitive method (Clin. Chem. 26: 327--331, 1980) for serum iron, in which the color reagent Ferrozine is used, is modified and adapted to the Abbott ABA-100 discrete analyzer. The standard curve is linear to at least 10 mg/L and the method showed day-to-day precision (CV) of 2.4% for a 1.03 mg/L sample (n = 63) and 1.9% for a 2.13 mg/L sample (n = 63). Lower values were obtained than with the modified continuous-flow technique of Giovanniello et al., but the correlation was good (r = 0.98). Bilirubin and copper do not interfere; hemoglobin and gross lipemia interfere only slightly. The total iron-binding capacity, based on Ramsay's method, was evaluated with regard to the effect of adding various amounts of magnesium carbonate. Results led us to use a ratio of approximately 180 mg of magnesium carbonate to each 5 micrograms of excess iron added. Day-to-day, the method for total iron-binding capacity gave a CV of 3.1% for a 2.55 mg/L sample, 2.8% for a 3.63 mg/L sample.
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