Post-warming survival has fallen — vitrification, warming, storage or handling?

Good-quality blastocysts should survive warming ≥95% of the time and re-expand within 2–4 hours. If survival drops — or embryos survive but fail to re-expand, or lyse partially — the cause sits in one of three places: how they were frozen, how they were kept, or how they were warmed. Because freezing and warming can be months apart, the first job is to work out which.

Step one: which cohort is failing?

• Freezing date. Group the failed warmings by the month they were vitrified. If failures cluster in embryos frozen in a particular window — and embryos frozen before and after are fine — the problem is on the freezing side (technique, media lot, device lot, or an operator) at that time.
• Storage location. Same tank? Same canister? Then think storage.
• Warming date / operator. Failures across embryos of many freezing dates, all warmed recently → the warming side.

Ten minutes with the cryo register usually answers this and halves the investigation.

If it's the freezing side (that vitrification window)

• Media lot — equilibration and vitrification solutions: lot change, expiry, storage temperature, opened bottle age. Vitrification media are unforgiving of storage excursions.
• Timing — equilibration time and, critically, time in the vitrification solution before plunging (typically under a minute; over-exposure is toxic, under-exposure leaves too much water). A stopwatch on the bench, or not; a new operator; a rushed session with several straws.
• Loading — volume around the embryo on the device: too much fluid slows cooling and forms ice. Device lot (some batches vary).
• Plunge — direct into LN₂, no vapour phase pause; LN₂ level in the plunging vessel; devices left in air a second too long.
• Embryo state at freezing — expanded blastocysts collapsed (laser or pipetting) before vitrification survive better; a policy change here shows up as survival change. Poor-quality embryos frozen under a more permissive policy lower the crude rate.
• Operator — split survival by who froze. Not to blame — to retrain.

If it's the storage side

• LN₂ level history for the tank — any period below the safe level? Any alarm event? Was the tank moved, topped up late, or has the fill schedule been missed over a holiday? Vapour-phase storage at the top of a low tank warms devices without anyone noticing.
• Transfers between tanks — how long out, and in what.
• Device type — open systems exposed to LN₂ during a contamination event.
• Tank condition — frost on the neck, unusual boil-off rate, a vacuum failing. A tank losing vacuum shows as rising LN₂ consumption weeks before it fails.

If it's the warming side

• Warming solution temperature — the first (thawing) solution must be at 37°C in the well when the device enters; a plate that says 37 but reads 33 with a probe is a classic cause. Check with a calibrated probe.
• Speed — hesitation between LN₂ and the first well; the device must move fast to avoid re-crystallisation.
• Timing in each dilution step; media lot and expiry.
• Handling — pipette diameter, aspiration force, moving a partially re-expanded blastocyst too early.
• Post-warm culture — medium, incubator, and time given before assessment; premature judgement of "not survived" on a collapsed embryo that would have re-expanded.
• Operator — again, split by person; look for who is new or returning.

Assessing survival honestly

Agree a definition and record it: fully intact / partial lysis (% cells) / degenerate; re-expanded at 2 h / 4 h; hatched. A shift from "intact" to "partial lysis" is a signal even when the headline survival rate looks fine.

Deciding and closing

Match the failing cohort to the side of the problem, then to the change nearest in time. Corrective action might be a media lot, a stage calibration, a retraining, or a tank taken out of service. Preventive: daily LN₂ level log with a minimum line, probe-check of warming plate before each session, cryo register that records operator, device lot and media lot at freezing, and survival by freezing month reviewed quarterly. Effectiveness: ≥95% survival with normal re-expansion over the next 20–30 warmings.