MP45-20: Variability in testosterone measurement between radioimmunoassay (RIA), chemiluminescence assay(CLIA) and liquid chromatography-tandem mass spectrometry(MS) among prostate cancer patients on androgen deprivation therapy (ADT).

Raj Tiwari*, Katherine Lajkosz, MohamedBaker Berjaoui, Yazan Qaoud, Toronto, Canada, Clive Woffendin, Portland, OR, Patrick Caron, Chantal Guillemette, Quebec City, Canada, Neil Fleshner, Toronto, Canada

Introduction: Monitoring testosterone (T) levels is increasingly being recommended to guide the effectiveness of ADT in the treatment of advanced prostate cancer (PCa). T levels of less than 20 ng/dl (0.7 nmol/L) on therapy have been associated with better outcomes with some clinicians advocating medication switch for patients who do not achieve this level. Three main assays for T measurement exist including RIA, CLIA and MS. CLIA and RIA are commonly used worldwide, however MS is regarded as the reference standard. We set out to determine the discordance rates of T measurements amongst men on ADT.

Methods: A retrospective McCain GU biobank (MGB) database review of PCa men on lutenizing hormone-releasing hormone (LHRH) monotherapy for 3 or more months was conducted. Patients with exposure to second line hormone agents or chemotherapy were excluded. Corresponding serum samples were identified and split in triplicate for measurement via all three assays. Observational data was reported and T measurements were compared analyzing for variability looking for categorical concordance. Over and under-estimation rates were calculated.

Results: 95 patients were included with a mean age of 70 (50-92) years. 80 (88%) patients were on LHRH agonist therapy. Mean ADT duration was 24.1 (3-144) months. Mean T levels were significantly different at p<0.001 with MS at 11.4 (0.1-282) ng/dL, CLIA at 23.4 (20-204) ng/dL and RIA at 15.1 (0.1-170.5) ng/dL. 95% of patients had T =20ng/dL by MS and CLIA as compared to only 80% by RIA. After subdividing into T categories of =20, 20-50 and =50ng/dL concordance analysis showed that 4.3% and 18.9% of ST measured by MS would have a different category result when remeasured by CLIA (Kappa 0.84) or RIA (Kappa 0.50) respectively. 16.8% of T measured by CLIA would also have a different category result when remeasured by RIA (Kappa 0.58). Intra-class correlation coefficient between all 3 measures was 0.83 (95% CI 0.77-0.88). CLIA and RIA overestimated T in 66% of patients with T <20 ng/dL measured by MS. Conversely CLIA and RIA underestimated T in only 2.4% with T>20ng/dL measured by MS.

Conclusions: There is significant variability in T measured with RIA, CLIA and MS. CLIA and RIA overestimated T levels in majority of patients leaving a concern of misdiagnosing true castrate patients instead as being inadequately treated. Clinicians should be mindful of variability in T measurements by assays when using them for decision making among PCa patients on ADT.

Source of Funding: Internal funding from University Health Network