. 2021 Jan 26;10:7. doi: 10.12703/r/10-7

Table 2. Summary of biochemical and clinical predictors of sperm retrieval in non-obstructive azoospermia.

Hormonal factors
Follicle- stimulating hormone (FSH)	FSH is secreted by the pituitary to stimulate Sertoli cells, which support the spermatogenic process of germ cells in men. An elevated FSH level has been associated with impaired spermatogenesis (Figure 1). Hence, some studies have proposed that a high level of FSH could be a predictor of SRR in mTESE^73,74. Yang et al. performed a meta-analysis of 11 studies that investigated FSH as a predictor of SRR prior to cTESE/mTESE. The AUC value of 0.72 was obtained after pooled analysis, suggesting FSH to be a moderate predictor of SRR⁷⁵. In contrast, Corona et al.⁷⁶ reported no correlation between FSH levels and SRR after pooling 117 studies. These inconsistencies in the literature may be explained by differences in surgical technique given that Yang’s meta-analysis included studies predominantly using cTESE (20/21), whilst Corona’s meta-analysis included 56 studies using cTESE and 43 studies using mTESE. Indeed, Ramasamy et al. observed in a large retrospective study of 792 men with NOA undergoing mTESE that SRRs were not associated with FSH levels⁷⁷.
Inhibin B	Inhibin B is produced by Sertoli cells and acts as a negative feedback regulator of FSH secretion. Therefore, inhibin B is a marker of spermatogenesis. High inhibin B levels in the serum or seminal plasma have been proposed to be an independent predictor of SRR⁷⁸. In a cohort of 403 NOA men, higher inhibin B levels were associated with successful SRR. The reported sensitivity was 77.9% and specificity was 91.58% (mixed cohort of mTESE and cTESE)⁷⁹. Yet Vernaeve et al. observed in a cohort of 185 NOA men that inhibin B was a poor discriminator of successful SRR (cTESE) with a sensitivity of 44.6% and a specificity of 63.4%. Using ROC analysis, the authors observed an AUC of 0.51 for the inhibin B concentration of 13.7 pg/ml⁸⁰. Moreover, a meta-analysis comprising 32 studies reported that serum inhibin B had a sensitivity of 0.65 and a specificity of 0.83 for predicting sperm retrieval in cTESE⁸¹.
Composite markers	Given the conflicting data regarding the utility of FSH and inhibin B individually as predictors of SRR, there have been attempts to combine both hormone levels to increase the predictive power of these markers. Von Eckardstein et al.⁸² observed that together serum FSH and inhibin B levels were a more sensitive predictor of the state of spermatogenesis than alone. However, collectively, these hormones predicted SRR from cTESE with a sensitivity of 75% and specificity of 73%. Similarly, Vernaeve et al. performed a ROC analysis utilising both inhibin B and FSH and reported that the AUC of inhibin B in men with an FSH level of <25 and ≥25 IU/l (the best threshold value for discriminating successful and unsuccessful cTESE was 25 IU/l) was 0.53 and 0.50, respectively⁸⁰. Boitrelle et al. combined testicular volume with serum FSH and inhibin B levels in a tri-variable predictive algorithm to predict cTESE outcomes⁸³. The authors reported that a score utilising these parameters was able to predict a successful cTESE with a positive likelihood ratio of +3.01. A score value of less than 18.5 correlated with a successful cTESE in 77.4% of cases and live birth rate in 41.8% of cases. Moreover, this value was also predictive of a sperm yield of greater than 100 spermatozoa in 91.1% of cases.
Summary	Within the context of the current literature, we cannot advocate a specific biochemical marker as a predictor of successful SRR owing to conflicting data.
Clinical factors
Patient age	Amer et al.¹⁷ investigated predictors of “difficult” (those with long durations, multiple biopsies required, and reduced SRR) sperm retrieval operations (mixture of cTESE and mTESE). The authors reported that histological pattern, FSH level, and testicular volume were not attributable. However, older age was a significant predictor and the mean age (SD) in more difficult operations was 39.4+/–7.95 compared to 32.75+/–6.76 (P <0.05) in those deemed less difficult. Moreover, the duration of infertility was noted to be a significant discriminator with the more difficult operations having a mean (SD) duration of 9.8+/–6.1 years compared to 5.2+/–3.8 years in those less difficult (P <0.05). Gnessi et al. observed that younger age was predictive of shorter procedure duration and faster recovery time. However, this study reported that age was not predictive of cTESE outcome after multivariable analysis⁸⁴.
Smoking	In a prospective study of 64 NOA men undergoing cTESE, tobacco use was observed to be an independent negative predictor of SRR (odds ratio 0.269, P = 0.045)⁸⁵.
Testicular volume	Testicular volume has been reported to be a predictor of SRR. Corona et al. observed in a meta-analysis comprising 117 studies (both mTESE and cTESE) that testicular volume was the only significant predictor of successful SRR. ROC curve analysis for a testicular volume of >12.5 ml predicted a SRR of >60%⁷⁶. However, caution must be applied to the above finding, as severe testicular atrophy does not exclude successful mTESE outcomes. Bryson et al. reported a SRR of 55% with a testicular volume of <2 ml, 56% with a testicular volume of 2–10 ml, and 55% with a testicular volume of >10 ml in 1,127 patients⁸⁶.
Cryptorchidism	A history of cryptorchidism was not associated with SRR in NOA men. Raman and Schlegel reported that the SRR (mixed cohort of mTESE and cTESE) was 74% in the cryptorchid cohort (n = 35) and 58% in the non-cryptorchid cohort (n = 274)⁸⁷. Barbotin et al. observed no significant difference in SRR for cTESE between unilateral (60%) or bilateral cryptorchidism (66.2%), P = 0.353⁸⁸.
Procedural factors
Previous failed TESE attempts	Friedler et al. reported that, in repeated cTESE procedures, the successful SRR was 33/39 (85%) and sperm could be found in men undergoing their fifth cTESE. Furthermore, there were no differences in fertilisation, implantation, or clinical pregnancy rate from sperm acquired from the first procedure and subsequent procedures⁸⁹. Moreover, Kalsi et al. observed that repeat mTESE had no impact on SRR, as 40% of those with previous failed cTESE or TESA procedures were successful on repeat attempts⁹⁰.
Embryological factors	The embryological extraction process has been reported to impact on SRR. Studies have reported that the addition of enzymatic treatment coupled with the conventional mincing method of testicular tissue confers a higher sperm yield^22,91.
Surgical factors	There is evidence that there is a learning curve for mTESE. Ishikawa et al. compared the SRR for a surgeon’s first 50 mTESE procedures compared to the subsequent 50 procedures. There were no differences in the patient clinical factors between the two cohorts, but the operation times were shorter and the SRR was higher in the later operation group (P <0.05)²¹.

AUC, area under the curve; cTESE, conventional (open) biopsies testicular sperm extraction; mTESE, microdissection testicular sperm extraction; NOA, non-obstructive azoospermia; ROC, receiver operating characteristic; SD, standard deviation; SRR, sperm retrieval rate; TESA, testicular sperm aspiration.