The issue of safety of biological fluids often arises not only among laboratory employees, but also among home owners animals for whom veterinarians have prescribed tests in specialized centers. Blood serum represents the liquid part of the blood, devoid of fibrinogen, and the reliability of future biochemical studies depends on the conditions of its maintenance. If the material is not delivered immediately, but requires time for transportation or waiting for the procedure, proper storage becomes a critical factor.
The temperature of the refrigerator is the determining parameter that affects the stability of the chemical compounds in the sample. An ordinary household refrigerator maintains a temperature in the range from +2 to +6 degrees Celsius, which creates acceptable conditions for short-term rest of the biomaterial. However, it is important to understand that native whey (not deep-frozen) begins to degrade faster than preserved analogues, and requires strict time control.
In this article we will look in detail at how long you can keep the test tube on the shelf of the refrigerator, what processes start in the liquid during cooling and how to avoid mistakes that could ruin the sample. Compliance with these rules ensures that the test results are accurate and do not require repeated blood donations, which is especially important for small or weakened animals.
Factors affecting the stability of the biomaterial
The stability of serum in the refrigerator depends on many variables that are often ignored during independent transportation. The first and main enemy is temperature fluctuation. Every time you open the refrigerator door, the temperature inside changes dramatically, which can trigger irreversible protein denaturation reactions. For laboratory purposes, this means the appearance of artifacts in the analyzes.
The second important factor is the integrity of the test tube and the absence of contact with other substances. Serum is highly hygroscopic and is capable of absorbing odors, as well as vapors of volatile substances if they are stored nearby. Hemolysis (destruction of red blood cells) can be exacerbated if the tube is not positioned correctly, which will lead to the release of intracellular contents into the serum.
It is also worth considering the initial quality of separation. If the blood was not centrifuged and separated from the cell mass in time, the processes of glycolysis and enzymatic activity continue even at low temperatures. In this case, the shelf life of the material is reduced significantly.
- 🌡️ Sudden temperature changes with frequent opening of the refrigerator door accelerate the deterioration of protein fractions.
- 💉 The presence of clots or partial destruction of cells (hemolysis) makes the sample unsuitable for precision biochemistry.
- ⏳ The time elapsed from the moment of blood collection to placing it in the cold directly affects the overall shelf life.
⚠️ Attention: If the serum has acquired a pinkish or reddish tint, this is a sign of severe hemolysis. Using such material for enzyme tests (ALT, AST, LDH) will give falsely high results.
Optimal temperature conditions and storage periods
The answer to the question of how long blood serum can be stored in the refrigerator depends on what specific temperature range you can provide. A standard household refrigerator maintains a temperature of about +4°C. Under such conditions, native whey retains its physicochemical properties for a limited time.
The usually recommended shelf life of whey in the refrigerator is no more than 48 hours (2 days). After this time, processes begin to change the concentration of glucose, reduce the activity of certain enzymes and change the electrolyte balance. For most biochemical indicators, a 24-hour interval is the “gold standard” of safety.
If longer storage is required, the refrigerator is not suitable - a freezer is required. However, even in the refrigerator, it is important to place the test tube correctly. It is best to place it in the back of the shelf, where the temperature is most stable, and avoid placing it on the door, where changes are most noticeable.
Below is a table showing the dependence of the preservation of various indicators from the time spent in the cold:
| Analysis parameter | Stability at +4°C | Recommended maximum |
|---|---|---|
| Glucose | Low (decreasing) | 2-4 hours |
| Protein fractions | High | up to 7 days |
| Enzymes (ALT, AST) | Medium | 24-48 hours |
| Electrolytes (K, Na) | Average (growth of K+) | 24 hours |
Differences between native and frozen whey
It is important to clearly distinguish between the concepts of “native” and “frozen” serum, since the rules for their use depend on this. Native serum is a liquid obtained after blood clotting and centrifugation, which has not been frozen. It is this that most often has to be stored in the refrigerator before delivery to the clinic.
Frozen serum, on the contrary, is stored at temperatures below -18°C or even -70°C in specialized freezers. In this state, biological processes practically stop, and the material can be stored for months. However, a single defrosting in the refrigerator already transfers it to the native category, and the return journey without loss of quality is often impossible.
Why can’t you re-freeze whey?
When re-frozen, ice crystals form, which destroy the molecular structure of proteins and hormones. This leads to precipitation and complete unsuitability of the sample for enzyme-linked immunosorbent assay (ELISA).
If you keep the test tube in the refrigerator, you are actually counting the lifetime of the native material. Enzymes remaining in the serum in microscopic quantities continue to work, albeit more slowly. Therefore, the longer the test tube sits, the more the real picture of the patient’s health is distorted.
- ❄️ Native serum in the refrigerator is chemically active, the decay processes are slow but constant.
- 🧊 Frozen serum is inert, but requires strict adherence to the cycle “one freeze - one defrosting."
- 📉 After 48 hours in the refrigerator, even visually transparent serum may be unsuitable for hormonal studies.
Rules for transportation and packaging of test tubes
Even if you know exactly how long blood serum lasts in the fridge, improper transportation can ruin all your efforts no. The test tube should be tightly closed with a paraffin or plastic cap. The use of cotton plugs in modern vacuum systems is not allowed, as they can allow moisture and microbes to pass through.
For transportation, use a thermal container or at least a tight bag with refrigerant (the refrigerant should not touch the test tube to avoid local freezing). In a household refrigerator, it is better to place the test tube in a clean plastic glass or container with a lid to protect it from accidental impact or contamination.
☑️ Rules for preparing for storage
Labelling is a mandatory element. The tube or label on it must indicate the date of blood collection, centrifugation time (if known), and refrigeration time. Without this data, the laboratory may refuse to accept the material, as it will not be able to guarantee the reliability of the result.
⚠️ Attention: Never store tubes with serum near food, especially meat or fish. This violates sanitary standards and creates a risk of cross-bacterial contamination.
Visual signs of serum unsuitability
Before taking the material to the laboratory, it is necessary to conduct a visual inspection. Healthy whey is usually clear and straw-yellow in color. Any deviations from this norm may indicate spoilage or improper storage in the refrigerator.
If you notice cloudiness, this may indicate the development of bacterial flora or loss of lipid fractions (if the animal ate fatty food before donation, although the serum should be obtained on an empty stomach). The appearance of flakes or sediment is a clear signal that the proteins have denatured or the growth of microorganisms has begun.
Color changes also play a role. A red tint indicates hemolysis, brown indicates a high level of bilirubin or oxidation, and green indicates a high content of bile pigments. Although some color changes are associated with disease, sudden cloudiness during storage most often indicates a violation of the temperature regime.
- ☁️ Cloudiness of the medium indicates bacterial growth or crystallization of salts/fats.
- 🩸 Pink or red color indicates destruction of red blood cells (hemolysis).
- 🌫️ The appearance of a “ring” or film on the surface may indicate contamination.
Mistakes made during storage
The most common mistake is storing the test tube on the refrigerator door. There, the temperature is constantly “walking” due to opening, which creates stress for the biomaterial. Thermal shock can lead to precipitation of proteins, even if this is not immediately noticeable visually.
The second mistake is storage together with the cell mass. If the blood is not centrifuged within 2-3 hours after collection and the serum is not removed into a separate clean tube, the cells will begin to break down, releasing potassium and enzymes into the medium. In this case, storing the sample in the refrigerator will no longer save the sample from distorting the results.
The third mistake is an attempt to “reanimate” frozen serum by rapid heating. If the whey accidentally freezes in the refrigerator (for example, on the back wall, where it is colder), it should not be heated in hot water or in the microwave. Slow thawing at room temperature is the only acceptable, although not ideal, option.
It is also a mistake to think that adding preservatives at home will solve the problem. Any foreign substances introduced into the test tube outside the laboratory will make the analysis impossible due to chemical interaction with the analyzer reagents.
Is it possible to store blood serum in the freezer of a regular refrigerator?
Technically, it is possible if the freezer keeps a stable temperature of -18°C. However, household refrigerators often have an auto-defrost function, during which the temperature rises. For critical tests, it is better to use laboratory freezers or deliver the material immediately.
What to do if the serum becomes cloudy after refrigeration?
If the cloudiness is caused by crystallization of fats (lipemia) after eating, the sample may not be suitable for many tests. If cloudiness occurs during storage, this is a sign of bacterial growth. You cannot donate such material; you will need to take a second blood sample.
Does light affect the serum in the refrigerator?
Yes, some components, for example, bilirubin and vitamin B12, are sensitive to light (photolabile). It is better to store the test tube in an opaque container or wrapped in foil, even inside the refrigerator, especially if there is an internal light.
How long can the serum be at room temperature?
Without refrigeration, native serum remains stable for only 6-8 hours. After this, rapid changes in the concentration of glucose and enzymes begin. Therefore, a refrigerator is required if delivery takes more than 2 hours.
Is it necessary to shake the test tube before analysis after refrigeration?
Yes, after removing it from the refrigerator, the test tube must be carefully mixed (inverted 3-5 times) to equalize the concentration of substances throughout the entire volume, since fraction separation could occur during cooling. You cannot shake sharply!