A bivariate radioimmunoassay for arginine vasopressin and the synthetic antidiuretic agent 1-deamino-8-D-arginine vasopressin (desmopressin).

Pairs of radioimmunoassays, each of which include a two-dimensional matrix of standards, have been previously employed to resolve specificity problems in steroid immunoassay. In this study the bivariate radioimmunoassay principle has been applied to simultaneous measurement of plasma antidiuretic hormone, arginine vasopressin, and the synthetic antidiuretic agent 1-deamino-8-D-arginine vasopressin (desmopressin), by utilizing two arginine vasopressin antisera which show significantly different cross-reactivities with the synthetic analog. Data processing consists of mathematical representation of two curved dose-response surfaces followed by solution of this pair of nonlinear simultaneous equations for the unknown arginine vasopressin and desmopressin concentrations. Details of numerical procedures are given in the Appendix. The assay appears entirely adequate in terms of sensitivity, accuracy, and precision for measurement of these antidiuretic agents in clinical samples. No evidence of significant covariance in estimated concentrations could be detected but precision of estimation is (not unexpectedly) a function of the concentration of both agents. The plasma disappearance half-time of desmopressin (probably the second of a biphasic disappearance) was estimated as 37 min in one normal subject, which is in good agreement with a previously reported value of 30 min.

A sensitive radioimmunoassay for measuring plasma anti-diuretic hormone in man.

A radioimmunoassay has been developed which permits the measurement of plasma antidiuretic hormone (arginine vasopressin (AVP), in the normal and subnormal range. Plasma was first extracted with octadecasilyl-silica to remove non-hormonal immunoreactivity, then assayed using a sensitive antibody and mono-iodinated AVP. After centrifugation of blood, we found AVP levels to be highest in the lowest layer of plasma--presumably because platelets are a reservoir for the hormone. In normal volunteers the range of plasma AVP was 0.8-14.3 pmol/l, and water loading generally lowered AVP levels. Appropriate increments in measured AVP were observed during intravenous infusion of the peptide, and during insulin-induced hypoglycaemia. Extremely low levels were found in cranial diabetes insipidus, whereas in some patients with bronchial carcinoma and hyponatraemia, AVP values were elevated. The sensitivity of the method and its practicability should make it useful both in clinical medicine and in research.

Influence of erythrocyte iodothyronine-binding proteins on radioimmunoassay of thyroxin in dried blood spots.

Three erythrocyte proteins, one identified as hemoglobin, bind thyroid hormones. Using a dextran/charcoal radioimmunoassay for thyroxin in dried blood spots, we demonstrate that such binding differs with the buffer used. Barbital, phosphate, and borate buffers significantly enhance the binding more than glycine and tris(hydroxymethyl)methylamine buffers. Binding is not affected by agents commonly used to inhibit thyroxin binding to serum proteins. A highly significant nonlinear direct relationship between sample storage (temperature and duration) and increased thyroxin-erythrocyte binding is documented, together with an associated decrease in assayed concentrations of thyroxin. However, concomitant serial measurement of thyroxin with polyethylene glycol and combined double-antibody/polyethylene glycol radioimmunoassays produced no evidence of interference by erythrocyte proteins in the radioimmune reaction. We conclude that erythrocyte proteins act only as low-affinity secondary binders in radioimmunoassay for thyroxin.

A screening programme for congenital hypothyroidism: preliminary results.

Highly sensitive and precise radioimmunoassays for thyroxine (T4) and thyrotropin (TSH) in dried blood spots on filter paper cards have been developed and are used to screen newborn infants for congenital hypothyroidism. Blood spot TSH levels are measured in samples for which blood spot T4 levels fall in the lower 10 to 15 percent. There was a low recall rate of approximately one infant in every 550 screened. During a 17-month period 5225 infants have been screened for congenital hypothyroidism and two cretins have been detected. Due to very early diagnosis, both infants were commenced on T4 replacement therapy before the age of three weeks.

Blood-spot thyrotropin radioimmunoassay in a screening program for congenital hypothyroidism.

We describe a highly sensitive and precise radioimmunoassay for thyrotropin in dried blood spots on filter paper cards. In a screening program for congenital hypothyroidism, blood-spot thyrotropin concentrations are measured in infants whose blood-spot thyroxine concentrations are in the lower 10%, and this strategy has reduced the recall rate from 1.7% (thyroxine assay alone) to 0.17%. Thyrotropin assay samples consist of discs 4.5-mm in diameter, containing about 6 microL of blood, punched from blood spots. By appropriate attention to assay conditions, a mean least-detectable thyrotropin concentration equivalent to 2.5 milliunits/L plasma has been achieved. Concomitant measurement of thyrotropin by plasma and blood-spot assays in 91 subjects yielded a Spearman rank correlation coefficient of 0.9732. An analysis of variance of the distribution volume of thyrotropin in blood spots and a covariance analysis of factors affecting blood-spot thyroxine results are presented.