Sperm Function and Sperm Preparation Techniques
Core concepts
Sperm function testing probes what the basic analysis cannot: membrane integrity, acrosomal status, mitochondrial activity, DNA fragmentation, and capacitation. Understand the physiology behind each assay and its limitations-DNA fragmentation assays (SCSA, TUNEL, comet) measure different things and are not interchangeable. For preparation, know density-gradient centrifugation (silica or iodixanol colloids) and swim-up, their selection principles (motility, density, morphology), and their effects on recovery, DNA integrity, and reactive oxygen species. Understand when to use each: swim-up for good-quality samples with high motility; density gradient for oligozoospermic or viscous samples and for removing debris and leukocytes.
Traps
Questions confuse the mechanism of swim-up (self-selection by active migration) with density gradient (separation by buoyant density). Watch for claims that preparation "improves" DNA integrity universally-gradients can enrich for intact DNA but do not repair damage. Another trap: assuming a functional assay predicts IVF outcome; most have poor positive predictive value. Also be alert to oxidative stress from prolonged centrifugation, high-speed spins, and leukocyte contamination, and to the fact that cryoprotectant addition and removal require controlled rates.
Worked example / rule of thumb
A sample with 10% progressive motility and heavy debris: choose density gradient, not swim-up, because swim-up requires sufficient motile sperm to migrate. Rule of thumb: minimize centrifugation time and force, keep processing at physiologic temperature, and record pre- and post-preparation recovery so you can audit technique.