Evaluation and comparison of two fully automated radioassay systems with distinctly different modes of analysis.

Two fully automated radioimmunoassay systems with batch and sequential modes of analysis were used to assay serum thyroxine, triiodothyronine, and digoxin. The results obtained were compared with those obtained by manual methods. The batch system uses antibody coated tubes while the sequential system uses immobilized antibody chambers for the separation of bound from free ligands. In accuracy, both systems compared favorable with the established manual methods, but the sequential system showed better precision than the batch system. There was a statistically significant carryover of thyroxine in the sequential system when there were at least six-fold differences in the concentrations of thyroxine in adjacent samples, but the carryover was not significant in the batch system. Compared with the batch system, the sequential system has a shorter throughtime for individual samples (time from aspiration of the sample to the printout of results) but a longer interval for final overall printout of assay results (lower throughput).

Nonspecific binding as a source of error in thyrotropin radioimmunoassay with polyethylene glycol as separating agent.

We investigated the effects of nonspecific binding on thyrotropin values obtained by radioimmunoassay in which polyethylene glycol is used as precipitant. Differences in nonspecific binding among individual samples were significant (F-test, p less than 0.001, range 5.5 to 14.1%). Non-specific binding and total serum protein were directly correlated (r = 0.472, n = 59; p less than 0.001). Nonspecific binding increased with increasing concentrations of globulins but showed no relation to albumin concentration. If globulin concentration was less than 15 g/L, precipitation of the antigen--antibody complex by polyethylene glycol was incomplete. The mean value for thyrotropin in sera from 67 healthy subjects was 2.7 (SD 0.3) milli-international units per liter (milli-int. unit/L) without individual serum nonspecific binding correction, significantly (p less than 0.005) higher than that with nonspecific binding correction (1.6, SD 0.1, milli-int. unit/L). Evidently, inter-sample variations in nonspecific binding may cause significant errors under these conditions, which can be minimized by taking into account the individual nonspecific binding of each serum sample.

A statistical method for determining normal ranges from laboratory data including values below the minimum detectable value.

Determination of normal ranges from laboratory data containing undectable values is a frequently encountered problem in the radioimmunoassay of peptide hormones. In the past, such determinations usually have been based on the mid-point method or the one-end Winsorized method. A graphic method involving the use of probability paper has also been reported. We propose that the maximum-likelihood estimation is a more appropriate statistical method for the determination of normal range from this type of data (Type I censored data). With this method, the mean and standard deviation, and hence the tolerance limits, can be estimated. We used the maximum-likelihood estimation method to determine the normal range of serum thyrotropin values obtained from 93 healthy subjects, based on a log normal distribution. Although the serum thyrotropin content was undetectable in 14% of the subjects, a normal range could be calculated. Using tolerance limits for 95% coverage of the population with 90% confidence, we calculated the normal range of thyrotropin to be 0.51-5.75 milliunits/L, with a mean value of 1.71 milliunits/L, and predicted that 91.4% of undetectable serum thyrotropin values will fall within the normal range.