Thawing sounds like the easy part. The bottle arrives on dry ice, you need it liquid, so you set it out on the bench and get on with something else. That habit is the source of most of the questions we get about cloudy serum, floating particles and lots that "used to work". Serum is a protein solution, and how it comes back from frozen decides what you have when it does. This is the way we thaw our own serum and plasma, and the reason behind each step.
Before you start
Start with the label. Our frozen serum and plasma ship on dry ice and are stored at -20 °C or below; complement products at -70 °C or below. If the bottle arrives soft or the dry ice is gone, note the condition and tell us the same day, before you thaw anything.
Take out only what you will use. A 500 mL bottle thawed for a 5 mL experiment has to be refrozen, and every freeze-thaw cycle costs you something. If you know you will work in small volumes, plan to aliquot the whole bottle on this first thaw (step 3 below) so it is the only time the bulk ever thaws.
Keep the cap on and the bottle closed until it is fully liquid and you are at the bench or in the hood.
Step 1: thaw in the refrigerator
For most serum and plasma the refrigerator is the right tool. Put the bottle at 2 to 8 °C overnight. A 100 mL bottle is liquid by morning; a 500 mL or 1 L bottle can take a day or more, so start it early. Swirl the bottle gently each time you pass the fridge. As ice melts, salts and proteins concentrate in the first liquid at the bottom of the bottle, and mixing as it thaws keeps that gradient from setting into a precipitate.
Slow thawing is gentle on labile proteins and gives you a whole bottle at fridge temperature, which is where you want it for aliquoting anyway. The only reason not to use it is time.
Step 2: or thaw in a 37 °C water bath
When you need the serum today, use a water bath set to 37 °C, not warmer. Blood banks thaw frozen plasma at 30 to 37 °C, and thawing hotter than that damages and precipitates plasma proteins (PMC). Thawing above 40 °C without mixing is a reliable way to make a precipitate inside the bottle.
Keep the water level below the cap. A water bath at body temperature is a good place for microbes to live, and a submerged cap carries bath water onto the threads and into the hood. Transfusion services put the plasma bag inside a sealed plastic bag for the same reason, and pull it out as soon as the contents are visibly liquid, normally within thirty minutes (ANZSBT).
Swirl the bottle every ten minutes or so. The moment the last piece of ice is gone, take the bottle out. Serum left sitting at 37 °C afterwards can grow a calcium phosphate precipitate that looks like contamination, and heat-labile components keep degrading the whole time it sits there. Wipe the outside of the bottle with 70% ethanol before it goes into the hood.
Step 3: mix, then aliquot
Once the bottle is fully liquid, swirl it until it looks uniform. Do not vortex or shake it hard. Foaming denatures protein at the air interface and does nothing the swirl does not already do.
Then split it: working aseptically, dispense the serum into single-use tubes sized for how you actually use it, whether that is 1 mL, 15 mL or 50 mL. Write the product code, lot number and date on every tube. Pick tubes with caps that seal at -80 °C; a cap that cracks in the freezer is a lost aliquot. The aliquots go straight back to the label temperature. Serum stored in aliquots is only ever thawed once more, at the bench, in the volume you need.
If you will use the thawed serum within a few weeks, you can hold the bottle at 2 to 8 °C instead. Our product pages give the window for each product (for most sera it is four weeks). After that, what is left should be discarded rather than refrozen.
Step 4: freeze the aliquots, and thaw each one only once
Every freeze-thaw cycle is a small insult. Ice forms, the remaining liquid gets more concentrated and its pH shifts, and proteins have to rehydrate on the way back. Most bulk serum components tolerate a handful of cycles: a study that took sera through ten freeze-thaw cycles found the common chemistry analytes largely stable, but total protein, albumin, bilirubin and lactate dehydrogenase did move (PMC). Cytokines and other low-abundance proteins are less forgiving, with some changing measurably within five cycles (PMC). Complement activity falls with every cycle. Because you rarely know in advance which component your assay depends on, the safe rule is one cycle for the bulk and one for each aliquot.
Keep the aliquots in a freezer without an automatic defrost cycle. A frost-free freezer warms itself periodically, which is a slow freeze-thaw cycle applied to everything on the shelf.
Cloudy serum, floaters and cryoprecipitate
Some haze or a few flocculent particles after thawing are normal. They are mostly fibrin, lipoprotein and calcium salts that came out of solution while the bottle was frozen or thawing. In our experience neither the haze nor the flocculent material changes how the serum performs. The fix is to thaw with regular mixing in the first place. If a precipitate has already formed, warm the bottle briefly to 37 °C and swirl; most of it goes back into solution. What does not can be left to settle and the serum decanted, or removed by aseptic filtration through a 0.45 µm filter if your application needs a clear product. Filtering does cost you some protein, so only do it when you need to.
What is not normal: a strong odor, a color change, turbidity that increases while the bottle sits at room temperature, or gas under the cap. Those point to microbial contamination, not cryoprecipitate, so do not use the bottle; contact us with the lot number and what you saw.
Complement is the exception
Complement is the one serum product that wants a fast thaw and no time at all in the refrigerator beforehand. Take the vial from -70 °C straight to a 37 °C water bath, swirl until the last ice disappears, then move it to an ice bucket and use it within the working day. Aliquot what you will not use immediately and refreeze at -70 °C; do not refreeze the working vial. The hemolytic titer drops with each freeze-thaw cycle, and time at room temperature costs activity too. The storage rules for complement, sera and lyophilized products are covered in storage, shelf life and freeze-thaw handling.
Plasma
Frozen plasma thaws the same way as serum, with one caution. Thawing plasma very slowly at 1 to 6 °C encourages cryoprecipitate, the fibrinogen and Factor VIII fraction that transfusion services deliberately harvest by cold thawing (PMC). For coagulation work, thaw plasma in the 37 °C water bath with mixing and use or aliquot it promptly. For plasma used as a matrix or diluent, the refrigerator is fine.
That is the whole method. Refrigerator overnight, or a 37 °C bath with the water below the cap; swirl as it thaws and take it out the moment the last ice is gone; aliquot on that first thaw and thaw each aliquot once, so the bulk bottle is never refrozen. Never shake it, never microwave it, never leave it on the bench. Complement is the one product that skips the refrigerator: straight from -70 °C to the bath, then onto ice for the day.
If a lot has had a rough thaw, or a freezer excursion, and you are not sure it is still usable, ask us with the lot number and what happened. For complement products we can advise on re-titering before you decide.