What causes sudden deterioration in fertilisation rates?

[Verified against the Vienna Consensus 2017 for indicator definitions] The consensus provides the diagnostic frame: several indicators exist specifically to interpret variation in fertilisation rate, and reading them together localises the problem. Indicators to examine as a set: Normal IVF fertilisation rate — competency ≥60%, benchmark ≥75%. Polyspermy rate — should be <6%. Values above this should be reported and investigated. Polyspermy is needed to interpret any observed variation in normal fertilisation rate. 1PN rate — <5% IVF, <3% ICSI. This is a marker of a problem in gamete handling or culture conditions. Failed fertilisation rate, IVF — <5% in stimulated cycles. A marker of a problem in gamete quality (sperm function, oocyte activation, gamete receptors), sperm processing, or the number of spermatozoa used for insemination. The pattern tells you where to look. A rise in 1PN points to gamete handling or culture. A rise in polyspermy points to insemination concentration. A rise in complete failure points to sperm function or processing. A fall in normal fertilisation with all three others normal points upstream, toward oocyte quality and therefore stimulation. Note the consensus definition of reference indicators: RIs relate to the oocytes coming into the laboratory and are proxy indicators of the response to ovarian stimulation. A deterioration may not be a laboratory problem at all, and RIs are how you establish that. Investigation should also cover the physical plant — incubator gas and temperature calibration, media lot changes, consumable lot changes, air quality, and any change in personnel or workload. Monitoring frequency: ideally monthly, or a predetermined case number where caseload does not permit, with 30 cases suggested as a starting dataset. Laboratories should remain vigilant and respond promptly to unexpected fluctuations.