Blood serum is the liquid part of plasma, purified from fibrinogen, and is a critical biomaterial for a huge range of laboratory tests. The preservation of its biochemical and immunological properties directly depends on compliance with strict temperature conditions and time intervals from the moment of blood collection to the moment of analysis. Any violation of transportation or storage conditions can lead to denaturation of proteins, changes in enzyme concentrations and, as a result, false diagnostic results.
In domestic conditions or in a small medical office, the question often arises about the permissible time for a test tube to remain in a standard refrigerator. The answer to this is not a definite number, as it depends on the type of study being conducted, the presence of preservatives and the tightness of the primary container. Understanding the physicochemical processes occurring in a biological fluid during cooling allows us to minimize the risks of damage to a valuable sample.
In this article we will analyze in detail the regulated storage periods, the influence of temperature fluctuations and specific requirements for various types of analyzes. You will learn why temperature regime is a key factor and what mistakes are most often made when trying to store material in a home refrigerator.
Physico-chemical processes in whey during cooling
When whey is placed in an environment with a temperature of about +4°C the rate of metabolic processes in cellular elements, if they remain, and the activity of enzymes are significantly reduced. This allows you to “preserve” the state of the sample for a certain time, preventing the breakdown of glucose and changes in potassium levels. However, a complete cessation of chemical reactions does not occur, and the degradation of some unstable compounds continues, although at a slower rate.
A critical point is the risk of hemolysis and moisture evaporation. Even in a closed test tube during long-term storage, microcondensation can occur or, conversely, drying out of the contents at the neck, which changes the concentration of substances. Protein denaturation If improperly cooled or repeated thawing (re-thawing), it makes the sample unsuitable for enzyme immunoassay.
What happens to enzymes during storage?
When whey is stored at +4°C, the activity of enzymes such as LDH (lactate dehydrogenase) and AST may change. LDH tends to increase, while alkaline phosphatase activity, on the contrary, decreases. This dictates strict time limits for biochemical tests.
It is important to consider that serum is not an inert liquid. The processes of lipid oxidation and interactions between components continue in it. That is why laboratory standards strictly regulate the maximum holding time of the material before and after centrifugation.
Standard storage periods at a temperature of +2...+8°C
For most routine biochemical and immunological studies, blood serum can be stored in the refrigerator at a temperature of +2 to +8 degrees Celsius for a limited time. The standard protocol is considered to be a period of no more than 48 hours (2 days). During this time, the structure of most proteins and the concentration of electrolytes remain stable.
If the study cannot be carried out within two days, storage at the temperature of a household refrigerator becomes risky. The processes of bacterial contamination (if sterility has been compromised) and chemical degradation begin to increase. For hormones and specific markers, this period can be reduced to 24 hours or even less.
- 🩸 Glucose: in whole blood, the glucose level falls quickly, but in separated serum at +4°C it is stable for up to 48 hours.
- 🧪 Enzymes: ALT and AST are stable for up to 2 days, while CPK (creatine phosphokinase) requires analysis in the first 24 hours.
- 🛡️ Hormones: thyroid hormones and cortisol can be stored for up to 2 days, but freezing is preferable for sex hormones.
It should be remembered that that a household refrigerator often does not provide a stable temperature. Opening the door, placing the test tube against the wall of the freezer or in an area blown with cold air can lead to local freezing or, conversely, heating. The critical threshold for the stability of most serum components is considered to be a period of 24 hours, subject to constant temperature control.
Long-term storage: freezing and deep cold
When it comes to preserving whey for more than 48 hours, freezing is the only safe method. At a temperature of -20°C, biological processes practically stop, which allows the material to be stored for 3 to 6 months without significant loss of properties. For long-term archiving, for example for scientific purposes or for retrospective analyses, temperatures of -70°C or lower are used.
However, the freezing and thawing process must follow strict rules. Sudden changes in temperature or re-freezing of an already thawed sample are strictly prohibited. Ice crystals formed during freezing can damage the molecular structure of proteins, and when repeated, the destruction becomes irreversible.
When defrosting, place the test tube in the refrigerator (+4°C) overnight or use a water bath at room temperature, but do not forcefully heat it. After thawing, the serum must be carefully mixed, avoiding foaming, and centrifuged again if a precipitate has formed.
The influence of the type of test on the shelf life
Not all tests are equally demanding on storage conditions. Some indicators are extremely labile and change literally before our eyes, while others remain stable for days. Understanding this difference is necessary for planning the logistics of delivering biomaterial to the laboratory.
For example, for coagulation studies (clotting), serum is not used at all, plasma is needed there, and the storage time is calculated in hours. For serology (search for antibodies to infections), the material is more stable. The hormonal profile requires special attention: insulin and some peptide hormones are destroyed faster than steroid hormones.
| Type of study | Max. period at +4°C | Max. period at -20°C | Note |
|---|---|---|---|
| Biochemistry (general) | 48 hours | 3 months | Avoid hemolysis |
| Hormones thyroid gland | 48 hours | 6 months | Stable when frozen |
| Tumor markers | 24 hours | 3 months | Requires rapid processing |
| Viral markers (Hepatitis, HIV) | 7 days | 1 year | High stability |
As can be seen from tables viral markers have high stability, which allows them to be transported under normal conditions for a week. At the same time, tumor markers require the fastest possible delivery or freezing.
Rules of transportation and packaging
Even if you plan to store the serum in the refrigerator for a short period of time, the delivery stage from the collection point to the storage location plays a decisive role. The tube must be tightly closed with parafilm or a special cap to prevent evaporation and contamination. Transportation is carried out in a vertical position in special tripods.
A thermal container with cold elements is used for transportation. It is important that the tubes do not come into direct contact with ice or dry ice, as this will cause local freezing of the serum and hemolysis. There must be an air gap or layer of insulating material between the test tube and the cold element.
- 📦 Use polystyrene foam containers to maintain temperature.
- 🌡️ Use temperature recorders to monitor conditions en route.
- 🚫 Avoid shaking and direct sunlight.
In a home laboratory or when collecting tests at home, the patient should immediately place the container in the refrigerator, in the middle shelf area, away from the door. Temperature conditions should be constant, without defrosting cycles.
Typical mistakes and risks of material damage
One of the most common mistakes is storing whey in the “legs” of the refrigerator or on the door, where the temperature fluctuates (fluctuates) every time it is opened. This results in the material being technically refrigerated, but actually being exposed to temperature shocks that accelerate spoilage.
⚠️ Attention: Never store open tubes of serum near food. Biological material is potentially infectious, and its storage in a household refrigerator with food is prohibited by sanitary standards due to the risk of cross-contamination.
Another mistake is an attempt to speed up the process of blood sedimentation by heating or, conversely, storage at a temperature below zero in a regular refrigerator without control. Crystallization of water in the serum leads to an increase in the concentration of salts in the remaining liquid, which changes osmolarity and destroys cells.
☑️ Checking storage conditions
Signs of serum unsuitability
How to understand that the serum has deteriorated and there is no point in conducting an analysis? Visually, high-quality serum is a clear or slightly opalescent yellow (straw) liquid. A change in color to red (hemolysis) or brown indicates the destruction of red blood cells, which makes it impossible to carry out many tests.
The appearance of flakes, turbidity, or a change in consistency (the serum becomes viscous) indicates bacterial growth or fibrin formation. Such material must be disposed of, as the analysis results will be false. Smell is also an indicator: the appearance of an unpleasant, putrid odor clearly indicates spoilage.
Laboratory technicians also evaluate the presence of microhemoconcretions or crystals. If the whey has been frozen and thawed incorrectly, it may become cloudy due to the precipitation of protein fractions. In such cases centrifugation before analysis may not save the situation if the proteins have already been denatured irreversibly.
FAQ: Frequently asked questions
Can serum be stored in a regular freezer? refrigerator?
Yes, it is possible if the temperature there remains stable around -18...-20°C. However, avoid the flash freezing area if there is one and do not open the chamber frequently. For long-term storage (more than a month), it is better to use a separate freezer to avoid defrosting cycles.
What to do if the whey becomes slightly cloudy after storage?
Slight opalescence may be normal (lipemia), but if cloudiness appears during storage, this is an alarming sign. The sample should be centrifuged. If the turbidity does not disappear or a precipitate appears, the material is most likely spoiled by bacteria or protein aggregates.
How long can the whey be kept at room temperature?
Without refrigeration, the whey (already separated from the cells) can be stored for up to 6-8 hours. However, if we are talking about whole blood before centrifugation, then the time is reduced to 2 hours, after which the active release of substances from the cells into the serum begins.
Does light affect the preservation of serum?
Yes, some components, such as bilirubin, vitamin B12 and some hormones, are sensitive to light (photolabile). Tubes containing such material should be stored in a dark place or opaque containers/foil should be used to protect from light.
⚠️ Attention: The terms and conditions stated are for reference only. Always refer to the instructions for the test system used in a particular laboratory, or the recommendations of the attending physician, since the requirements for the preanalytical stage may vary.