A steady-state gel filtration method on micro-columns for the measurement of percentage free testosterone in serum

Ramifications of variability in sex hormone-binding globulin measurement by different immunoassays on the calculation of free testosterone

Objective

Sex hormone-binding globulin (SHBG) is a glycoprotein which binds hormones such as testosterone. Around 97% of circulating testosterone is bound to SHBG or albumin and is therefore biologically unavailable; 2–3% of testosterone is free. Free testosterone is very technically challenging to quantify; in order to circumvent this problem, equations using testosterone and SHBG are used to estimate free testosterone. We decided to determine the effect of using different SHBG immunoassays on calculated free testosterone results.

Design

Anonymized surplus serum samples were analysed for SHBG on four different immunoassay platforms (Abbott Architect, Roche, Beckman and Siemens). The SHBG results were used to generate a Vermeulen calculated free testosterone.

Results

Beckman Access and Siemens Centaur both gave results close to the overall mean. Roche gave the highest SHBG concentrations with Abbott Architect producing the lowest results. Abbott Architect gave the highest calculated free testosterone results, followed by Beckman. Roche gave the lowest results. Sixty-five per cent of male samples had low calculated free testosterone and 27.5% of the females had high calculated free testosterone using the SHBG from the Abbott assay compared with 69% low male calculated free testosterone and 20% high female calculated free testosterone with the Roche assay.

Conclusion

Our results have shown significant differences in SHBG results produced by different analysers and subsequently the calculated free testosterone, which may affect result interpretation if method-specific reference ranges for calculated free testosterone are not used. Care should be taken to ensure reference ranges are appropriate for the analyser used to avoid misdiagnosis of hypo or hyperandrogenism, and ensure patients get the most appropriate treatment.

The measurement of androgens

A number of androgens are measured for clinical purposes. Most laboratories will run a testosterone assay but the requirement for other androgens may be too small for a laboratory to set up their own assay. In these cases samples would be sent to a specialized laboratory. In the routine laboratory testosterone is analyzed on automated systems but these lack the sensitivity to accurately measure the hormone in children and normal women. Many laboratories analyzed such samples by radioimmunoassay, but tandem mass spectrometry is now replacing these assays. This chapter provides simple methods for measuring the androgens in serum and saliva by radioimmunoassay and tandem mass spectrometry.

Late-onset male hypogonadism: clinical and laboratory evaluation

Late-onset male hypogonadism (LOH) is a clinical and biochemical syndrome associated with advancing age and characterised by low serum testosterone concentrations. An understanding of the physiology of androgens in the ageing man is essential for the appropriate diagnosis of LOH. Clinical assessment of androgen status relevant to clinical biochemists and chemical pathologists is outlined in this review. Laboratory investigations of androgen status in men are not without pitfalls and the authors highlight problems associated with measuring and calculating serum testosterone and its fractions, the interpretation of which can be problematic. Current clinical guidelines and recommendations regarding the diagnosis and monitoring of LOH are also summarised.

Sex Differences and Individual Differences in Cognitive Performance and Their Relationship to Endogenous Gonadal Hormones and Gonadotropins

Sexually dimorphic cognitive performance in men (n=42) and women (n=42) was related to testosterone, estradiol, progesterone, luteinizing hormone, follicle-stimulating hormone, and sex hormone binding globulin, measured in 10-ml blood samples collected between 0900 and 1030 and, among women, during the follicular phase of the menstrual cycle. Significant sex differences favored men on spatial tasks (Mental Rotation and Judgment of Line Orientation) and on an inhibition task and favored women on a verbal task (category fluency). However, there were no significant relationships between any of the hormones and cognitive performance, suggesting that there are few, if any, consistent, substantial relationships between endogenous, nonfluctuating levels of gonadal hormones or gonadotropins and these cognitive abilities in men or women.

Determination of free testosterone fraction of human serum by gel bead dialysis

The gel bead dialysis technique is an application of the steady-state gel filtration method. Preswollen gel beads are used as microdialysers. The gel is incubated for 3.5 h at 22 degrees C with 2.5 mL of serum and 3H-testosterone. During incubation, the equilibrium between free and protein-bound fractions is reached and the concentration of testosterone in the bead dialysate is equal to the free testosterone concentration in the incubation medium surrounding the beads. After incubation, the gel is separated from the medium by using a rigid filter and the radioactivity within the gel is counted. The effect on the assay of serum dilution and incubation time were studied. Within-batch imprecision (CV) was 3% (N = 30) and between-batch imprecision computed from lyophilized quality controls run over a one year-period, was 7%, 13% and 17% for free testosterone fractions (free over total) of 0.043, 0.032 and 0.021 respectively. An excellent correlation between this technique and the equilibrium dialysis method was found (r = 0.944, N = 59). Reference intervals were determined for men, women and pregnant women; mean +/- SD were 0.0216 +/- 0.0067 (N = 20), 0.0106 +/- 0.0020 (N = 20) and 0.0077 +/- 0.0017 (N = 19) respectively. Patients suffering from polycystic ovary syndrome had significantly higher free testosterone ratio (mean of 0.0285 +/- 0.0074, N = 20) than normal females.

Hyposexuality and epilepsy: a community survey of hormonal and behavioural changes in male epileptics

A survey of five group practices in South London identified 60 male patients currently undergoing treatment for epilepsy. Fifty-four agreed to participate in a detailed enquiry into aspects of their sexual activity and behaviour. Anterior pituitary and sex-hormone levels were measured. The epileptic patients were characterized by low levels of sexual activity and interest. Temporal-lobe and non-temporal-lobe epileptics were indistinguishable in these respects, but when the latter group was further subdivided into primary generalized epilepsy and focal non-temporal lobe epilepsy, the focal groups (both temporal lobe and non-temporal lobe) were more impaired. Hormonal analysis confirmed earlier hospital-clinic-based reports. In the epilepsy group, plasma free testosterone and percentage free testosterone values were decreased, sex hormone binding globulin, luteinizing hormone (LH) and follicle stimulating hormone (FSH) were increased compared to values in a normal healthy population. As association between the behavioural and hormonal indices of hyposexuality was shown, particularly for LH, but this was less apparent than in previous work.

A symmetric dialysis method for the determination of free testosterone in human plasma

We describe a symmetric dialysis (SYD) method for the determination of the fraction (fT), and the concentration (FT) of non-protein-bound testosterone (T) in human plasma. In SYD fT is estimated from the rate at which radiolabelled testosterone redistributes between two identical (undiluted) plasma aliquots separated by a dialysis membrane. We compared the effect of tracer contamination and sample dilution on FT as measured by SYD and equilibrium dialysis (EQD) in five plasma pools differing in Sex Hormone Binding Globulin (SHBG) and total T concentration. In addition, FT was calculated by computer simulation of the interaction between binding proteins and steroid hormones. With SYD relatively small decreases in FT were observed following two- to tenfold dilution (14.3 +/- 4.4%). A comparable decrease was calculated by computer simulation (13.3 +/- 2.2%). The apparent fall in FT as estimated by EQD was much larger (44.7 +/- 2.2%) and, contrary to assessment by SYD, this fall depended on the radiochemical purity of the 3HT-tracer. Moreover, since SYD allows the assessment of fT in undiluted samples, problems relating to the interference of buffer components with T-binding are circumvented. We conclude that SYD is to be preferred over EQD for the accurate and precise measurement of fT and consequently FT.

Direct assay for testosterone in saliva: relationship with a direct serum free testosterone assay

A direct non-extraction radioimmunoassay for salivary testosterone is described using a modified commercial kit procedure that is in use for total serum testosterone (T). Serum free testosterone was also measured by direct radioimmunoassay. A significant correlation (r = 0.83, p less than 0.01, n = 194) was obtained between salivary and serum free testosterone in matched serum and saliva samples over a wide range of concentrations. Within- and between-batch precision for the salivary testosterone method was 11% and 18%, respectively at a concentration of 170 pmol/l. Recovery of added T was 89% +/- 15% (mean +/- 2 SD) dilution of high samples showed parallelism. Salivary testosterone measured by direct radioimmunoassay offers a simple cheaper alternative to serum free testosterone measurement with the additional advantages of a stress-free non-invasive sampling procedure.

Plasma free testosterone--is an index sufficient?

Only about 50% of women who are clinically hirsute have a raised total plasma testosterone concentration; in those cases where the total testosterone is normal the free testosterone may, in fact, be raised. Since the measurement of free testosterone is tedious, workers have used an androgen index or a calculated free testosterone concentration from the measurements of total testosterone and sex-hormone-binding globulin. We have examined the correlation between measured free testosterone, a derived free testosterone and an androgen index in hirsute women, normally menstruating women and non-hirsute women on oral contraceptive therapy. These three measures of the free testosterone concentration in blood gave similar results in all cases.