Fertilisation rate has dropped — where to look, in order

A fertilisation rate that was sitting at 75–80% and is suddenly 55% is one of the commonest lab alarms — and one of the hardest to trace, because so many things touch an oocyte between collection and the 16–18 h check. This guide is a sequence, not a list: work down it, rule things out, and write each step in your CAPA as investigated and excluded. That record is worth as much as the answer.

First: is it real?

• How many cycles? Two or three poor cycles in a week can be patient mix. Look at at least 15–20 injected oocytes across several patients before calling a trend. The EmbScore Monthly Workup shows this by month; a mid-month check is fair when something feels wrong.
• Which rate? Compute 2PN ÷ MII injected (ICSI) or 2PN ÷ inseminated (IVF), not ÷ oocytes collected. A run of immature cohorts lowers the crude figure without a lab problem.
• Split by operator, by day, by incubator. Three minutes with the case log often points straight at a factor.
• Split by AMH / age band and by indication. A cluster of poor-prognosis patients (severe male factor, low responders) explains more than people expect. If the drop is uniform across all groups, think lab.

Second: timing

• Trigger-to-OPU and OPU-to-ICSI intervals. Injecting too early after denudation (before the oocyte has recovered) or very late (oocyte ageing) both cost fertilisation. Check whether theatre timings, a new OT list, or a staffing change has shifted the ICSI window.
• Denudation timing relative to OPU, and how long oocytes wait outside the incubator during it.

Third: the sperm side

• Preparation — new density-gradient lot, changed centrifugation force/time, a new technician. Check motility after preparation, not just before.
• Selection at the ICSI dish — PVP/hyaluronan drop age, sperm immobilisation technique (too gentle → no activation, too aggressive → damaged).
• A cluster of severe male factor / surgically retrieved sperm in the affected period.
• If IVF (not ICSI): insemination concentration, and whether a threshold for switching to ICSI/rescue ICSI is being applied.

Fourth: what the oocyte sits in

This is where most sudden changes hide, because lots and consumables change on a date you can look up.

• Culture medium lot — fertilisation/cleavage medium and the ICSI/handling medium. New lot on or just before the drop? Check the batch record and, if available, the manufacturer's certificate of analysis. Note that a good lot handled badly (temperature excursion in transit, left warm, opened too long) behaves like a bad lot.
• Oil lot — oil is often forgotten. Some lots are toxic to embryos; the sign is often both lower fertilisation and poorer early cleavage. Rule: one new variable at a time when opening new lots.
• Denudation enzyme (hyaluronidase) — concentration, exposure time, lot. Over-exposure damages the oolemma.
• Dishes, pipettes, tubing, gloves — a new plastics lot or a new brand of gloves can be embryotoxic. Any change of consumable within two weeks of the drop is a suspect.
• Water and washing if any in-house preparation exists.

Fifth: physical environment

• Heated stages and warm blocks — actual drop temperature (measured with a calibrated probe in a drop, not the display) at the ICSI microscope and stereo. A stage running 35°C or 38.5°C both hurt.
• pH / osmolality — bicarbonate media out of the incubator drift alkaline within minutes; check how long dishes are out and whether the ICSI dish is prepared under oil / in HEPES-buffered medium as intended. Osmolality of the ICSI dish creeping up (evaporation, small drops, low humidity, long sessions).
• Incubator — CO₂ set-point vs measured CO₂ (Fyrite / independent analyser), temperature at dish level, humidity, recent door-opening load, a new incubator commissioned without validation. See the registry entry for your model.
• Air quality — new paint, floor cleaning agents, alcohol wipes, a nearby renovation, a perfume-wearing visitor. VOCs suppress fertilisation and cleavage. Ask what changed in the building.

Sixth: technique

• ICSI technique — depth of injection, aspiration of ooplasm to confirm membrane breakage, oocyte orientation (polar body position), speed. Review with the operator using video where available. New staff or a returning colleague after leave are frequent, innocent explanations.
• Degeneration rate after ICSI above ~5–10% points at technique, pipette lot, or oocyte fragility (over-exposure to enzyme).

Deciding and closing

• Line every change up against the date fertilisation fell and the date it recovered. The change closest before recovery is your likely cause; the ones you tried that didn't help are your "excluded" list.
• Corrective action: what you changed back or replaced. Preventive action: how you'll stop it recurring — usually a lot-change log with one variable at a time, a daily QC sheet for stages and incubators, and a monthly KPI review.
• Effectiveness: fertilisation rate back in range for a full month. Attach the EmbScore summary.