Isotope dilution analysis using chromatographic separation of isotopic forms of the compound to be measured

Using progesterone, testosterone, androstenedione, 11-oxoprogesterone and 11 beta-hydroxyprogesterone as models, a new form of isotope dilution assay has been developed. A known mass of deuterium-labelled steroid is added to the serum sample. High-performance liquid chromatography is used to separate endogenous steroid from its deuterium-labelled form. After separation, the two forms of the analyte are quantitated using conventional methods: radioimmunoassay, enzyme-linked immunoassay and, where the concentrations are high enough, ultraviolet light absorption. The ratio of the amounts of the two forms of the analyte is used to calculate the amount of unlabelled material in the original sample. The assay principle is quite general. A variety of high resolution methods are available to separate isotopic analogues of the same compound. A number of detection methods can be used to quantitate the separated isotopic forms. Extension of this principle to other fields of interest in bio-medicine is discussed briefly.

Reliability of progesterone measurements in urine

A radioimmunoassay (RIA) of progesterone in urine is described. After the addition of labelled progesterone, morning urine was extracted with n-hexane adn the residue was either directly subjected to RIA, or chromatographed on celite prior to RIA. The progesterone from celite chromatography was radiochemically pure. RIA after chromatography was therefore considered valid. The non-chromatographed procedure resulted in overestimations, the degree of which was inversely proportional to progesterone content. The results obtained by the two procedures were well correlated (r = 0.88 and 0.93, for 2 different groups of samples.

Validation of radioimmunoassay for estradiol-17 beta by isotope dilution-mass spectrometry and by a test of radiochemical purity

Isotope dilution-mass spectrometry (ID-MS) was used as a reference method to determine the concentration of estradiol-17 beta (E2) in five different plasma pools (concentrations ranging from 0.040 to 65 nmol/l). The same plasma pools were also subjected to radioimmunoassay (RIA) using five different antisera of largely varying specificity. With the best antiserum (E), a direct RIA apparently gave accurate results (i.e. results statistically indistinguishable from those obtained by ID-MS) at all levels except the lowest one (0.040 nmol/l). It was shown, however, that the apparent accuracy of this RIA to some extent could be due to a lowering effect of lipids in the serum masking a lack of specificity of the antibodies. With the least specific antiserum (A), accurate results were obtained only after chromatography. However, in the assay of the lowest concentration of E2 with this antiserum there was a significant overestimation, even after chromatography. The other three antisera (B, C, D) of average quality gave accurate results in assays of plasma diethyl ether extracts in various numbers of the plasma pools tested, depending on their intrinsic specificity. This specificity was not correlated with the cross-reaction reported for individual antisera. ID-MS is difficult to use in most laboratories. We demonstrate here that the validity of a RIA may in this case be assessed by a relatively simple method, the test of radiochemical purity (RP-test). This test is based on the measurements of specific activity (e.g. dpm/pg) in small consecutive fractions of the chromatographic zone which is usually employed for the RIA.

Problems of validity in steroid immunoassays

A valid steroid immunoassay is the one which correctly describes the contents in unknown samples. Such an assay has to be accurate (devoid of bias) and precise. Assay accuracy can be assessed by a comparison of the results with those obtained by a reference method, e.g. gas liquid chromatography-mass spectrometry. Alternatively, the accuracy assessment can be made in terms of the purity of the compound assayed. The test of parallelism or the test of radiochemical purity can be used for this purpose. The former test, however, is not fully reliable. The degree of precision is of great importance especially when validity of individual measurements is considered. A low reproducibility (a high between-assay variation) leads to results of uncertain validity. An increased standardization and simplification of assays may result in a substantial improvement of the precision, and thus validity.

Influence of antisera, purification procedures, and tracers on validity of a testosterone radioimmunoassay

The levels of testosterone in pooled plasma from men and from women were measured by radioimmunoassay. Testosterone was isolated from plasma either by ether extraction (direct procedure) or by chromatography of the ether extract (chromatographic procedure). Three different antisera (all raised against testosterone-3-oxime albumin) were employed. An additional variable was the use of two tracers, [3H] testosterone and [125I] histamine testosterone. The latter tracer was employed in both a freshly prepared form and two months after labelling. The old tracer was used both in a nonpurified and repurified form. The results were compared with those given by one of the antisera using the assay with chromatography. The validity of this assay was verified earlier using a test of radiochemical purity. In the statistical evaluation of the data, it was found that the assays of both plasma pools gave the same results as the validated assay, even with two other antisera, but only when the chromatographic purification was used. The direct assays gave significantly increased values for testosterone in the male plasma pool with one antiserum, and in the female plasma pool with all three antisera. It is concluded that those assays maybe considered valid which give results statistically indistinguishable from those obtained by the valid assay. These were all chromatographic assays and, in the man, also two direct assays. The comparison of [3H] testosterone and [125I] histamine testosterone as tracers did not indicate any major differences in the validity of the assays. When the 125I-tracer was employed, a moderate (1.5-2 fold) increase of sensitivity was seen with all three antisera, and the antisera could be used in a 10-40 times higher dilution. Furthermore, it was possible to use [125I] histamine testosterone and obtain practically the same results after two months, whether repurified or not.