Vitrification has transformed what is achievable in assisted reproduction. Survival rates for vitrified embryos and oocytes in experienced UK laboratories now consistently exceed 90 percent, and frozen embryo transfer cycles in some programmes match or outperform fresh cycles for live birth rates. But headline survival rates tell only part of the story. Behind every percentage point sits a complex combination of factors – the quality of the consumable system used, the precision of the protocol followed, the consistency of the team applying it, and the infrastructure supporting storage from the moment a sample is frozen to the moment it is warmed.

This article covers what the published evidence says about vitrification success rates in UK IVF, what the most significant determinants of outcome are, and where the consumable and equipment decisions your laboratory makes directly affect the results your patients achieve.

What the Published Evidence Shows on Vitrification Survival Rates

The published literature on vitrification outcomes is now extensive. A landmark meta-analysis by Rienzi et al. (2017), published in Human Reproduction Update, found oocyte survival rates of 90.6 percent and fertilisation rates comparable to fresh oocytes in experienced vitrification programmes. Subsequent studies have confirmed that blastocyst survival rates following vitrification in established programmes typically fall between 90 and 97 percent, with some centres reporting higher figures.

The Human Fertilisation and Embryology Authority (HFEA) publishes annual data on frozen embryo transfer outcomes across UK licensed fertility clinics. The data consistently shows that frozen embryo transfers now account for the majority of IVF cycles in the UK, reflecting both the growth of freeze-all strategies and the confidence the clinical community has placed in the reliability of modern vitrification.

What the aggregate figures do not show is the variation between programmes. Survival rates in the published literature range from the low 80s to over 97 percent across different centres and protocols. That variation is not random. It is the product of specific, identifiable differences in how vitrification is performed and with what.

The Cryoprotectant System: Why Matched Kits Matter

The cryoprotectant solutions used in vitrification are not interchangeable between manufacturers, and the choice of kit is one of the most significant determinants of survival rate a laboratory can influence.

A vitrification kit is a matched system. The equilibration solution, the vitrification solution and the warming solutions are formulated to work together, with the osmotic properties of each step calibrated to work in sequence. The equilibration solution begins the dehydration of the cell gently. The vitrification solution completes this at higher concentration and under strict time limits, because high-concentration cryoprotectants are cytotoxic if exposure exceeds the validated window – typically 60 to 90 seconds. The warming solution then reverses the process, progressively diluting cryoprotectant while rehydrating the cell.

If warming solutions from a different manufacturer are used alongside a different vitrification kit, the osmotic steps at thaw may not match the dehydration that occurred during freezing. The result is osmotic stress on rehydration – cells can swell and lyse, producing post-thaw survival rates that are worse than they should be and that the laboratory may attribute to sample quality rather than protocol mismatch.

The CBS High Security Vitrification Kits available through Cryolab supply the complete matched system – equilibration, vitrification and warming solutions – in a single validated kit. The kits are available in multiple colours for colour-coded patient identification and traceability, and are fully compatible with the CBS storage system including visotubes and goblets. Browse the full vitrification kit range here.

The Carrier System: Open vs Closed and the Clinical Implications

The choice between open and closed carrier systems is a debate that has run through UK embryology for over a decade. Open carrier systems expose the sample directly to liquid nitrogen during plunging, achieving the highest possible cooling rates. The concern is the theoretical risk of cross-contamination between samples sharing the same liquid nitrogen bath, particularly relevant for patients with known transmissible infections.

Closed systems, of which the CBS High Security Vitrification Kit is the leading example in UK practice, vitrify the sample within a hermetically sealed ionomeric resin straw. The sample never contacts the liquid nitrogen directly. The cross-contamination pathway is eliminated entirely. HFEA guidance acknowledges both systems as clinically acceptable in UK licensed practice. For laboratories working with patients who have transmissible infections, or for those that have made a policy decision to use closed systems across all patients, the CBS High Security Vitrification Kit provides the validated closed-system option with an extensive evidence base from European and UK IVF centres.

Protocol Consistency: The Human Factor in Vitrification Outcomes

The most technically advanced vitrification kit in the world does not protect against inconsistent execution of the protocol. The timing of the vitrification solution step – the 60 to 90 second window between loading the sample and plunging into liquid nitrogen – is not a guideline. It is a biological constraint. Published research consistently identifies protocol consistency as a key predictor of vitrification survival rates. A study by Coticchio et al. (2014) in Human Reproduction demonstrated that inter-operator variability in vitrification technique was a significant source of outcome variation within programmes, independent of the kit used.

Laboratories with high survival rates share one characteristic above all others: they treat the warming step with exactly the same rigour as the freezing step, because post-thaw survival is determined by both ends of the process equally.

Training, standardisation and quality assurance systems for vitrification protocols are as important as the choice of consumable. Laboratories should audit survival rates per operator and per embryo stage, and validate any change in consumable supplier or kit format before implementing it clinically.

Storage Conditions: What Happens Between Vitrification and Warming

A vitrified embryo stored at -195.8 degrees C is biologically stable indefinitely, but only if storage conditions are maintained without interruption. Temperature excursions above the critical threshold of approximately -130 degrees C – even brief ones during an out-of-hours alarm event or a sample retrieval – can compromise samples that survived the vitrification step in excellent condition.

The quality of the storage vessel directly affects the safety margin available around access events. The CryoNest series of liquid nitrogen storage vessels from Cryolab is designed for the operational demands of busy IVF laboratories, with large capacity, low evaporation rates and organised rack systems that minimise the time and disruption of each sample access event. The CryoNest XL at 95 litres and the CryoNest XXL at 145 litres suit different programme volumes, both designed to maintain stable -195.8 degree C conditions with hold times that provide a genuine safety margin.

A backup vessel kept charged and ready is not optional in a licensed UK IVF setting. The capacity planning that goes into your vessel specification today directly affects the clinical safety of samples stored under the extended storage limits introduced by the Health and Care Act 2022. For product guidance specific to your vitrification programme, contact the Cryolab team or use CryoGPT for immediate answers.

References

Rienzi, L., Gracia, C., Maggiulli, R., LaBarbera, A.R., Kaper, R.F., Ubaldi, F.M., Vanderzwalmen, P. and Racowsky, C. (2017). Oocyte, embryo and blastocyst cryopreservation in ART: systematic review and meta-analysis comparing slow-freezing versus vitrification. Human Reproduction Update, 23(2), pp.139-155.

Coticchio, G., Bromfield, J.J., Sciajno, R., Gambardella, A., Scaravelli, G., Borini, A. and Albertini, D.F. (2014). Vitrification may increase the rate of chromosome misalignment in the metaphase II spindle of human mature oocytes. Reproductive BioMedicine Online, 29(1), pp.54-61.

Human Fertilisation and Embryology Authority (2024). Fertility treatment 2022: trends and figures. London: HFEA.

Health and Care Act 2022. London: HMSO. In force 1 July 2022.