Interference in a two-site immunoradiometric assay for thyrotropin in a child.

Unusual results from immunoassays and the role of the clinical endocrinologist

“…40 Use in the patient of biotin is rare but has been shown to influence enzyme immunoassay of TSH and free T4. 41 Raised calcitonin indicating medullary thyroid cancer Fine-needle aspiration 5 Raised prolactin level Confusion in diagnosis of prolactinoma [6][7][8] Increase in urine free cortisol Inappropriate investigations (due to use of topical steroids) 9 Elevated testosterone in women Inappropriate investigations 10 Low result Negative STAT hCG pregnancy Mismanagement of choriocarcinoma 11 False low digoxin results Dosing with digoxin increased to toxicity 12,13 Elevated LH and FSH levels Over-investigation 14,15 Low insulin level Confusion in diagnosis of insulinoma 16 Elevated and low results False thyroid hormone or TSH results Inappropriate thyroid treatment 14,17,18 False positive and negative troponin results Inappropriate investigation or treatment of myocardial infarct (MI)…”

“…The main technique currently in use for removing or identifying heterophilic antibodies is that of blocking with nonimmune serum from the same or different species as the antibodies used in the assay agents. 17,77,78,98,140 Patient samples frequently fail to show discrepant results when only sample dilution or antibody blocking tubes are used. In one study, 79% of samples with suspected interference showed identifiable interference in antibody blocking studies whereas only 21% demonstrated interference on dilution.…”

“…The main technique currently in use for removing or identifying heterophilic antibodies is that of blocking with nonimmune serum from the same or different species as the antibodies used in the assay agents 17,77,78,98,140 . Patient samples frequently fail to show discrepant results when only sample dilution or antibody blocking tubes are used.…”

Anomalies in the Assay of Parathyroid Hormone in Normocalcaemic Patients With Renal Stone Disease

“…The persistence of the apparent immunoreactivity following adsorption with charcoal suggests that the interfering factor is likely to be a high molecular weight substance. The hazards of circulating antibodies to animal immunoglobulins have already been noted in several immunometric assay systems (Clark et al, 1985;Cusick et al, 1985;Hunter & Budd, 1980;Prince et al, 1973) and must be considered here. A circulating antibody which interferes with the binding of PTH to the assay antiserum would be expected to produce elevated results in the N-terminal one-site IRMA systems.…”

“…Firstly, non-immune sheep IgG was included at a concentration of 20 mg/l in assay 2 to block potential interference from antibodies to sheep immunoglobulins. Secondly, anti-IgG antibodies do not appear to be species specific and it is often possible to prevent assay interference with IgG from a different species (Boscato & Stuart, 1986;Cusick et al 1985). Although the addition of bovine gamma globulin restored PTH levels to normal in assay 6, dilution of affected samples with excess foetal calf sera did not reduce PTH values in assay 2 to within normal limits.…”