Background: Existing conventional serum tumor markers (STMs) exhibit moderate performance for the detection of germ cell tumors (GCTs). Circulating miR-371a-3p has excellent performance in the detection of viable (non-teratoma) GCT pre-orchiectomy; however, its ability to detect occult disease is understudied. We refine the serum miR-371a-3p assay and expand upon our previous experience in this setting. Methods: We compared performance of raw (Cq) and normalized (∆Cq, RQ) values from prior assays (n=93), and validated interlaboratory concordance by aliquot swapping. Revised assay performance was determined in a cohort of 32 patients suspected of occult retroperitoneal disease. Assay superiority was determined by comparing resulting receiver-operator characteristic (ROC) curves using the Delong method. Pairwise t-tests were used to test for interlaboratory concordance. We defined an indeterminate range as the mean of the low Cq peak from controls ± 2 SDs. Results: Performance was comparable when thresholding based on raw Cq vs. normalized values. Calculated sensitivity and specificity were both greater than 0.9 in all cases and did not change appreciably across any of the metrics tested. Interlaboratory concordance of miR-371a-3p was high, but reference genes miR-30b-5p and cel-miR-39-3p were discordant. We identified for the 1^st time an indeterminate range; Introduction of an indeterminate range of Cq 28-35 with a repeat run for any indeterminate improved assay accuracy from 0.84 to 0.92 in a group of patients suspected of occult GCT. Conclusions: We recommend that serum miR-371a-3p test protocols are updated to a) utilize threshold-based approaches using raw Cq values, b) continue to include an endogenous (e.g., miR-30b-5p) and exogenous non-human spike-in (e.g., cel-miR-39-3p) microRNA for quality control, and c) to re-run any sample with an indeterminate result.