The use of zona-free aged unfertilized human oocytes as a predictor for successful subzonal insemination

Objective To examine the reliability of the zona-free aged (1 day old) human unfertilized oocyte sperm penetration assay for assessing sperm fertilizing ability and to determine the predictive value of this assay for subsequent subzonal insemination (SUZI) outcomes. Design A total of 253 unfertilized oocytes from total fertilization failure patients and from good fertilization rate (>70%) patients in standard IVF were inseminated with donors' spermatozoa, and penetration rates were compared in each group. Two hundred seventy-two unfertilized oocytes from total fertilization failure, poor fertilization ( 30%) were inseminated with husbands' spermatozoa, and penetration rates were compared between the three groups. In 29 patients, the results of the zona-free assay performed in previous IVF were compared with the fertilization rates of subsequent SUZI. Results In the zona-free assay using donors' spermatozoa, there was no difference in penetration rates between the two groups (109/122, 89.3% versus 114/131, 87.0%). Penetration rates using partners' spermatozoa were positively correlated with fertilization rates in standard IVF (total fertilization failure 34/75, 45.3%; poor fertilization 56/77, 72.7%; normal fertilization 108/120, 90.0%). There was a significant difference in fertilization rates after SUZI between the patients with negative and positive penetration in zona-free assay (4/53, 7.5%, versus 54/174, 31.0%). Conclusion The zona-free human oocyte assay may primarily reflect sperm fertilizing ability. This assay also could be a reliable predictor for subsequent SUZI outcome.