Diluted human serum as a suitable diluent in the preparation of bilirubin reference solutions.

From a concentrated solution of bilirubin in dimethylsulfoxide (about 13 mmol/1, standardized spectrophotometrically) and a serum diluted 1/3 (whose native bilirubin is lowered by exposure to phototherapy lamps) a bilirubin-enriched serum is prepared (bilirubin concentration about 130 mumol/1 = 7.5 mg/dl) wherein all the bilrubin is bound to albumin. Such an enriched serum behaves in the diazo-reaction in the same way as a bilirubin-enriched serum prepared with undiluted serum and can, therefore, be used as a working standard in serum bilirubin assays.