Zona Binding Test

The zona binding test (also called the hemizona assay or sperm-zona binding test) is a specialized assay used primarily in research settings to evaluate a sperm's ability to recognize and bind to the zona pellucida, a key step in natural fertilization. Each egg is cut in half; one half is incubated with the patient's sperm and the other with sperm from a fertile donor as a control.

A reduced binding ratio compared to the control suggests a functional sperm defect beyond what standard morphology or motility tests capture. The test has been used to identify which patients with unexplained fertilization failure might benefit from ICSI.

It is not routinely available in clinical practice due to scarcity of non-viable donor eggs and has largely been supplanted by ICSI for all borderline cases.