Sperm Processing Improves Motility in High Uric Acid Men

A retrospective cohort study published in Basic and Clinical Andrology reports that density gradient centrifugation, a standard laboratory technique for preparing sperm for intrauterine insemination, is associated with a greater improvement in progressive sperm motility among men with hyperuricemia compared to those with normal uric acid levels. The study, conducted at a single center in China, analyzed data from 490 couples undergoing their first intrauterine insemination cycle between April 2017 and February 2025. The researchers found that while the procedure significantly boosted the percentage of progressively motile sperm in both groups, the increase was statistically larger in the group with elevated serum uric acid, suggesting a specific benefit for this metabolic subgroup.

Hyperuricemia, defined in the study as a serum uric acid level greater than 420 micromoles per liter, is a condition characterized by high levels of uric acid in the blood. The study authors noted that the prevalence of this condition in China rose from 8.5% in 2001 to 18.4% in 2017. Beyond its association with gout, hyperuricemia has been linked to male infertility through mechanisms such as oxidative stress and metabolic dysregulation. The research team aimed to determine whether this metabolic state impairs sperm motility and whether density gradient centrifugation could specifically reverse such damage.

The study included 200 cycles from men in the hyperuricemia group and 290 cycles from men in the control group, who had serum uric acid levels at or below 420 micromoles per liter. Participants were couples with male or unexplained infertility undergoing their first intrauterine insemination in a natural cycle protocol, meaning no ovulation-inducing medications were used.

Before any laboratory processing, the baseline percentage of progressively motile sperm was significantly lower in the hyperuricemia group than in the control group. The mean percentage was 39.55% in the hyperuricemia group compared to 41.76% in the control group, a difference that remained significant after adjusting for multiple comparisons. This baseline disparity supports the premise that elevated uric acid levels are associated with reduced initial sperm motility, consistent with prior literature suggesting oxidative stress plays a role in this impairment.

Density gradient centrifugation involves layering semen over a solution of varying density and spinning it to separate motile sperm from immotile cells, debris, and seminal plasma components. In this study, the procedure used a two-step gradient with a 90% bottom layer and a 45% upper layer, followed by washing and resuspension in a culture medium. This process is widely used in assisted reproductive technology to select high-quality sperm for insemination. The researchers measured semen parameters before and after this processing to quantify the change in motility.

Following density gradient centrifugation, the percentage of progressively motile sperm exceeded 90% in both the hyperuricemia and control groups, with no significant difference between the two post-treatment values. However, the magnitude of the improvement was the key finding. The increase in the percentage of progressively motile sperm was significantly greater in the hyperuricemia group, with a mean increase of 52.34% compared to 50.29% in the control group. This difference, though numerically small, was statistically significant, indicating that the procedure may specifically counteract the motility impairment associated with hyperuricemia.

The authors suggested that this specific improvement may be related to the ability of density gradient centrifugation to remove reactive oxygen species and pathological metabolites from the seminal plasma. Hyperuricemia is associated with reduced antioxidant capacity and elevated oxidative stress, which can damage sperm membranes and energy metabolism. By removing these harmful elements and providing energy substrates through the culture medium, the centrifugation process may alleviate the specific metabolic defects caused by high uric acid levels. It is important to note, however, that this study did not directly measure oxidative stress markers, so this mechanistic link is inferred from prior experimental studies rather than direct evidence from this cohort.

Despite the specific improvement in sperm motility, the study did not find a significant difference in clinical pregnancy rates between the two groups. The clinical pregnancy rate was 11.0% in the hyperuricemia group and 13.4% in the control group, a difference that was not statistically significant. This finding suggests that improving sperm motility alone is insufficient to enhance pregnancy outcomes in this population. The authors noted that other factors, such as sperm DNA integrity and female reproductive factors, may play a dominant role in determining fertilization success and pregnancy outcomes.

The lack of improvement in pregnancy rates may be attributed to several factors. First, once post-processing motility exceeds a certain threshold, other determinants of pregnancy likely dominate. Second, hyperuricemia may cause persistent impairments to sperm functional competencies, such as capacitation or the acrosome reaction, which are not fully addressed by motility improvement alone. Third, the study had a high rate of missing data for female ovarian reserve markers, such as anti-Müllerian hormone, which could have masked potential benefits. Finally, the sample size for the hyperuricemia subgroup may have limited the statistical power to detect small differences in pregnancy rates.

The study has several limitations that should be considered when interpreting the results. As a retrospective cohort study, it is subject to selection bias and residual confounding. The extended study period of eight years may have introduced variability in clinical protocols and laboratory practices. The researchers did not measure sperm DNA fragmentation index or key sperm functional parameters, which limits the ability to fully explain why improved motility did not translate to higher pregnancy rates. Additionally, male body mass index data was not collected, which is a potential confounder for semen quality and metabolic health.

These findings provide insights into optimizing semen preparation strategies for men with hyperuricemia-associated infertility. The study suggests that density gradient centrifugation may be a targeted approach for this specific metabolic subgroup, offering a basis for individualized assisted reproductive technology protocols. However, the results do not imply that the procedure will improve pregnancy rates, and further research is needed to understand the broader impact of metabolic abnormalities on reproductive outcomes.

Future research directions suggested by the authors include functionalizing centrifugation solutions with reactive oxygen species scavengers to enhance the removal of oxidative stress mediators. They also proposed stratifying patients by baseline motility to personalize treatment and extending follow-up to track live birth rates rather than just clinical pregnancy. These approaches could help bridge the gap between improved laboratory parameters and meaningful clinical benefits for men with metabolic abnormalities.

The study’s statistical framework was designed to detect differences in the change of progressively motile sperm percentage, referred to as ΔPR%, between the two groups. A post hoc power analysis indicated that the sample size was sufficient to reliably detect the observed difference in this primary endpoint, with a statistical power of 0.83. In contrast, the power to detect differences in the secondary endpoint, clinical pregnancy rate, was only 0.22. This lower power is attributed to a smaller observed effect size and may explain why the study did not identify a statistically significant difference in pregnancy outcomes, despite the observed numerical variation between the groups.

Baseline characteristics such as age, duration of infertility, and type of infertility did not differ significantly between the hyperuricemia and control groups.

The laboratory protocol for density gradient centrifugation involved specific reagents and centrifugation speeds to ensure consistency. Semen samples were layered over a gradient solution and centrifuged at 300 × g for 15 minutes. The resulting sperm pellet was then washed and resuspended in a culture medium containing energy substrates such as pyruvate, lactic acid, and glucose. These substrates are intended to support sperm energy metabolism through glycolysis and oxidative phosphorylation, potentially compensating for energy supply defects associated with hyperuricemia. The procedure was performed using standardized operating procedures, with monthly calibration of counting equipment to maintain accuracy.

Pregnancy confirmation followed a two-step process. Preliminary screening was conducted 14 days after insemination using urine or serum beta-human chorionic gonadotropin tests. Positive screening results were confirmed via transvaginal ultrasound at five weeks post-insemination. A clinical pregnancy was diagnosed if a gestational sac was identified, while a biochemical pregnancy was defined by elevated hormone levels without a visible sac.

Subject of Research: Impact of density gradient centrifugation on sperm motility and pregnancy outcomes in men with hyperuricemia

Article Title: Density gradient centrifugation specifically improves sperm motility in hyperuricemia: evidence from intrauterine insemination cycles – retrospective cohort study

Article References: Peng, J., Wu, L., Li, Z., Huang, Q., & Li, L. (2025). Density gradient centrifugation specifically improves sperm motility in hyperuricemia: evidence from intrauterine insemination cycles – retrospective cohort study. Basic and Clinical Andrology, 35(1), Article 42. https://doi.org/10.1186/s12610-025-00292-z

Image Credits: AI Generated

DOI: 10.1186/s12610-025-00292-z

Keywords: hyperuricemia, sperm motility, density gradient centrifugation, intrauterine insemination, male infertility, retrospective cohort, oxidative stress, assisted reproductive technology, Density, gradient, centrifugation, specifically