Development and evaluation of a time-resolved immunofluorimetric assay for thyrotropin

The assay described in this article is based on microtitre plate technology; it employs an europium label. The streptavidin-biotin system has been used and all components are commercially available. The lower detection limit of the assay is below 0.003 mU/l; the standards are made up in newborn calf serum. Correlation with a commercially available immunoluminometric assay (Berilux--Behringwerke) was excellent (r = 0.92, n = 201 data pairs, range covered 0-10 mU/l). The regression line using a double logarithmic transformation was: (log y) = 0.91 (log x)-0.08. The assay precision at the clinical limits of decision--i.e. between hyper- and euthyroidism (0.2 mU/l) and eu- and hypothyroidism (4 mU/l)--was acceptable. The median coefficient of variation was 1.93% in the range 0.02-1 mU/l and 2.11% in the range 1.0-3.5 mU/l, with both values being determined from precision profiles using 214 and 188 data pairs respectively. Inter-assay coefficients of variation determined in over 30 consecutive assays were under 6% in the range 1.3-20 mU/l. From 201 sera measured in both assays, 194 sera gave clinically identical values; 8 sera gave clinically discrepant values. The assay has a large dynamic range covering a concentration range of above 5 decades, with the count ratio between the 100 mU/l standard and the zero standard being in excess of 4150:1. A high dose hook effect was first seen in excess of 500 mU/l. The maximum signal was achieved around 150 mU/l, which registered around 8 x 10(6) counts per second, a figure more than 8000 times higher than that in the zero standard.

The digoxigenin:anti-digoxigenin (DIG) technology--a survey on the concept and realization of a novel bioanalytical indicator system

A review is given on the novel non-radioactive digoxigenin:anti-digoxigenin (DIG) bioanalytical indicator system. After a general introduction on direct and indirect indicator systems based on previous non-radioactive indicator reactions as well as in vitro and in vivo amplification procedures the principle of the new digoxigenin:anti-digoxigenin technology is demonstrated. The novel system is based on the specific high-affinity interaction between the cardenolide digoxigenin from Digitalis plants and a digoxigenin-specific antibody coupled with a reporter group. A variety of methods for digoxigenin modification of nucleic acids, proteins and glycans are presented. In addition, various applications of the novel non-radioactive indicator system in a variety of direct or indirect detection approaches with either insoluble or soluble substrates are described. It is also shown that with these applications alternative reaction formats are used which are partly characterized by additional amplification steps.

Evaluation of enhanced luminescence immunoenzymometric assays (LIA) for ferritin and free T4

The Amerlite enhanced luminescence immunoenzymometric assays (LIA) for ferritin and free T4 (FT4) have been evaluated. The LIA assay system employs a two-site binding approach in which one antibody is coated on the interior of plastic microtiter wells and another antibody, or antigen, is tagged with horseradish peroxidase (HRPO). Signal, proportional to the amount of patient analyte, is produced when the HRPO catalyzes the oxidation of luminol resulting in light production. The light signal is increased in magnitude by the incorporation of an enhancing agent which also greatly prolongs the lifetime of the signal. Light production is monitored by an automated luminometer which uses a stored calibration curve to calculate patient results. FT4 within-run coefficients of variation (CVs) ranged from 2.7% to 6.6%; day-to-day CVs from 6% to 15.5% for three levels of control. Sensitivity is 0.03 ng/dl. T3 concentrations up to 8 ng/ml produced no cross-reactivity. Dilutional linearity was exhibited. Lipemia and icterus produced positive interferences. Linear regression analysis of Amerlite and radioimmunoassay (RIA) FT4 values yielded the following values: Amerlite = 0.63 RIA - 0.2, r = .87, n = 79. Ferritin within-run CVs ranged from 4.9% to 10.5%; day-to-day CVs from 5.8-12.5% for three levels of control. Sensitivity is less than 1 ng/ml. Recoveries ranged from 93% to 106% (av. = 99.4%). Dilutional linearity was exhibited. Lipemia caused a positive interference; hemoglobin caused a negative interference. Linear regression analysis of Amerlite and RIA ferritin values yielded the following values: Amerlite = 1.0 RIA + 11.1, r = .96, n = 55.

Analytical goals for quantities used to assess thyrometabolic status

Analytical goals for imprecision derived from data on intra-individual variation for total T4, free T4, total T3, free T3 and thyrotropin (TSH) are coefficients of variation (CV) less than or equal to 2.5, 4.7, 5.2, 3.9 and 8.1%, respectively. If total T4 is used to monitor replacement therapy with thyroxine, a more stringent goal of CV less than or equal to 1.4% is appropriate. For those analytes for which biological variation data are not available, analytical goals may be derived either from reference intervals or from the 'state of the art' as judged from the performance of a stated proportion of laboratories participating in an interlaboratory quality assessment scheme. Analytical goals for imprecision for reverse T3 and thyroxine-binding globulin derived from reference values are CV less than or equal to 10.7 and 7.2%, respectively. The goal for inaccuracy is that there should be none. Statements regarding the detection limit of an assay should be replaced with information about the range of concentrations over which specified goals for imprecision are met. If goals are not achieved at concentrations which are used for clinical decision making the 95% confidence limits of the extreme values of the 'working range' should be calculated using the relevant imprecision. Improved analytical performance will result in better between-laboratory comparability and eventually allow the use of universally applicable reference values.

Clinical utility and cost-effectiveness of sensitive thyrotropin assays in ambulatory and hospitalized patients

In either an ambulatory or a hospitalized patient setting, a normal serum sensitive thyrotropin (TSH) value is strongly suggestive of euthyroidism if the patient has intact hypothalamic-pituitary function and is not receiving drugs known to suppress pituitary TSH secretion. In stable ambulatory patients, an abnormal sensitive TSH value is strongly suggestive of clinical or subclinical thyroid hormone excess or deficiency, which can be confirmed by a free thyroxine (T4) index (FT4I) and evaluation for antimicrosomal antibody (AMA) as a marker of autoimmune thyroid disease. In a hospitalized patient, an abnormality in sensitive TSH or FT4I does not necessarily indicate a thyroid problem but may merely reflect a nonthyroidal illness or glucocorticoid or dopamine treatment. A thyrotropin releasing hormone (TRH) test may be needed to diagnose hyperthyroidism in a hospitalized patient with a basal sensitive TSH level of less than 0.1 microU/ml because a detectable TRH response contraindicates hyperthyroidism whereas hyperthyroid patients with nonthyroidal illness have the expected absent response. In a hospitalized patient, hypothyroidism must be diagnosed on the basis of both a high TSH level and a low FT4I because an isolated high TSH value may merely reflect the recovery phase of a nonthyroidal illness. No clinical urgency exists for establishing a diagnosis of subclinical hypothyroidism in a hospitalized patient; definitive determination of thyroid status can be deferred until recovery and discharge.

An avidin-biotin amplified ELISA for thyrotropin in human serum. Its implication for routine determination of thyrotropin in developing countries

In developing countries the acquisition and use of radioisotopes with their short shelf lives poses enormous problems. The objective of this study was to develop a simple, sensitive, non-isotopic method for the measurement of TSH in human serum. A conventional sandwich enzyme-linked immunosorbent assay (ELISA) technique was modified to incorporate the avidin-biotin complex (ABC) in order to enhance the sensitivity of a solid-phase micro-ELISA for TSH. The lower limit of TSH detectability was 0.2 mIU/l. TSH levels ranged from 0.3 to 5.1 mIU/l in euthyroid subjects, 11.5-98.0 mIU/l in hypothyroid subjects and less than 0.2-2.1 mIU/l in hyperthyroid subjects. Sera from euthyroid post-menopausal or pregnant women yielded TSH levels within the normal range. The correlation between the TSH values obtained in the ELISA-ABC and a sensitive commercial immunoradiometric method was high (n = 59, r = 0.980, P less than 0.01). The simplicity and relatively high sensitivity of the procedure should make it a method of choice for TSH determination in small hospitals and those in developing countries.

Evaluation of five high-sensitivity American thyrotropin assays

We compared five commercial immunometric kits for thyrotropin (TSH) manufactured in the United States--Abbott, Diagnostic Products, Hybritech, Nichols, and Becton Dickinson--and one sensitive "in-house" radioimmunoassay for their ability to monitor TSH levels in patients with hyperthyroidism and during thyroid hormone suppression therapy. With these five methods, we measured TSH concentrations is serum specimens from the following categories of patients: euthyroid (N = 99), hyperthyroid (N = 51), hypothyroid (N = 50), and thyroid cancer and nodular goiter on thyroid hormone suppression therapy (N = 33). The immunometric methods were similar in assay sensitivity and precision; the in-house radioimmunoassay was less sensitive than all other assays studied. The simultaneous correlation of TSH values obtained from all diagnostic categories was more than 90% among the assays studied. The immunometric assays clearly distinguished hyperthyroid from euthyroid patients and quantitated TSH levels in the subnormal range.