Effects of haemolysis, urea and bilirubin on the precision of digoxin and insulin radioimmunoassays

The influence of haemolysis, uraemia and hyperbilirubinaemia on the radioimmunoassay for both digoxin and insulin has been investigated for five separation techniques (dextran/charcoal; coated tube; polyethyleneglycol 4000; sodium sulphite; double antibody). Recoveries, and intra- and interassay precision were calculated. It was demonstrated that even in serum samples with a rather high degree of haemolysis (haemoglobin up to 50 g/l) digoxin can be measured by using each of the five separation techniques without any significant interference. Visible haemolysis (haemoglobin above 200 mg/l) leads either to disturbance or to a complete failure of insulin radioimmunoassays with all separation techniques. This effect can be largely neutralized, and precision improved, by using N-ethyl-maleimide. With the exception of the coated tube separation technique the intraassay precision has a CV of < 10%, and the interassay CV is between 10 and 20%. Elevated urea concentrations interfered in the digoxin radioimmuno-assay only when the coated tube technique was used. The insulin radioimmunoassay, however was affected by high urea when either the double antibody or the coated tube technique was used. Here the intraassay precision also has a coefficient of variation < 10%, the interassay CV lying between 10 and 20%. Bilirubin influenced the digoxin test when the sodium sulphite separation was used, and it affected the insulin determinations with polyethyleneglycol 4000 and sodium sulphite. The intra- and interassay precision were however also around 10% and between 10 and 20% respectively. Compared with the interassay precision of 15% CV for digoxin and 13% for insulin for a pool-serum from blood donors, the decrease of interassay precision caused by haemolysis, uraemia and hyperbilirubin-aemia was insignificant. (orig.)