Preliminary results on the use of an antiserum to human parathyrin in a homologous radioimmunoassay.

A new antiserum (Ab Giselle) raised in sheep against extracted human parathyrin was evaluated and compared with an established antiserum (Ab S-478 VI) under several test conditions. The assay system contained an extracted 1--84 human parathyrin standard and a 1--84 bovine parathyrin tracer. The total assay time was 24 h and the main assay characteristics as follows: B0/T 0.28 +/- 0.02 and 50% intercept 553 +/- 47 U . 1(-1). The corresponding data for Ab S-478 VI were: B0/T 0.23 +/- 0.02 and 50% intercept 890 +/- 142 U . 1(-1). The normal range in 152 normocalcaemic volunteers (age range 16--67 years) was 10.6--423 U . 1(-1) (where 1 vial MRC reference preparation 75/549 for human parathyrin = 25 U), compared with 0--300 U . 1(-1) for Ab S-478 VI. With the new antiserum, differentiation between hypoparathyroid patients and those with normal function was often possible, and differentiation between normal and elevated levels, as in hyperparathyrinaemia, was very good. Correlation between Ab Giselle and Ab S-478 VI in 80 normal volunteers was positive (r = 0.450, p = 0.01) although the regression line showed that the antisera had different specificities (data for the regression line y = a + bx, a = 0.13, b = 0.55). Under the assay conditions, the association constant for Ab Giselle was 0.41 +/- 10(14) l . mol-1 in contrast to Ab S-478 VI which had a Ka for 0.53 x 10(10) l . mol-1 under identical conditions. Assays using Ab Giselle could be performed at room temperature, whereas those using Ab S-478 VI performed best at 0 degrees C. Preliminary results suggest that Ab Giselle is better for the routine assay of human parathyrin in serum than Ab S-478 VI, especially in the case of hypoparathyroid patients.