Shelf life of biological materials is a critical parameter in laboratory diagnostics. The accuracy of the result directly depends on how long and under what conditions the sample is kept in the cold. Patients often wonder how long a test tube can remain in the home refrigerator if it is not possible to donate it immediately.
The answer is not as simple as it seems, since different blood components and different types of tests require fundamentally different conditions. Temperature regime and transportation time can radically change the biochemical composition of serum or plasma. In this article we will analyze the physical and chemical processes occurring in the test tube after collection.
Exists critical rule: most tests for glucose and hormones cannot be stored in a household refrigerator for more than 2-4 hours without pre-treatment. Violation of this condition leads to false diagnoses. Next, we will consider in detail the dependence of the stability of indicators on time and temperature.
Physico-chemical processes in the test tube after collection
After the syringe needle is removed from the vein, Irreversible processes begin in a test tube. Blood cells continue to live and consume nutrients, even while outside the body. Hemolysis, or the destruction of red blood cells, can begin within a few hours, especially if the temperature regime is disturbed.
It is important to understand that a refrigerator is not an analogue of the conditions inside the human body. At a temperature of +4...+6°C, metabolic processes in cells slow down, but do not stop completely. Enzymatic reactions continue, which leads to a change in the concentration of analytes.
What happens to blood cells in the cold?
When cooled, the membranes of red blood cells become more rigid and prone to rupture. This releases intracellular contents (potassium, LDH) into the serum, which distorts the results of biochemistry.
In addition, moisture evaporates through the rubber stopper, which leads to thickening of the sample. The concentration of substances increases artificially, and the laboratory technician receives falsely high readings. That is why tightness and time are of paramount importance.
Standard storage times for whole blood
Whole blood is the least stable material for research. Without separation into fractions (serum or plasma), it quickly loses its diagnostic properties. For most clinical tests, whole blood has a shelf life of a few hours.
If you donate complete blood count (CBC) on an anticoagulant (in a tube with a purple cap), the sample is relatively stable. At room temperature it is good for up to 4-6 hours, and can be stored in the refrigerator for up to 24 hours, although this is not recommended for precise hematology.
However, whole blood cannot be stored for biochemical studies. It must sit at room temperature to form a clot, after which it is centrifuged. Only the resulting serum or plasma must be refrigerated.
The influence of temperature on the stability of analyzes
Storage temperature is a key factor. A standard household refrigerator supports a mode from +2 to +8°C. This is the optimal range for short-term storage of many specimens, but not all. When cooling, some enzymes, on the contrary, are activated or change their structure.
There is a concept cryolability. This is the property of some substances to break down at low temperatures. For example, von Willebrand factor or some types of cryoglobulins precipitate at +4°C. They cannot be cooled before delivery to the laboratory.
The table below shows approximate data on the stability of various analytes at different temperature conditions:
| Analyte | Room t° (20-25°C) | Refrigerator (+4°C) | Frozen (-20°C) |
|---|---|---|---|
| Glucose | 8 hours | 24 hours (with fluoride) | 3 months |
| Potassium | 2 hours | 48 hours | Not recommended |
| TSH (hormone) | 8 hours | 3 days | 1 month |
| Bilirubin | 1-2 hours (in the light) | 2 days (in the dark) | 3 months |
How as can be seen from the table, illumination also plays a role. Bilirubin is destroyed by exposure to light, so yellow-capped tubes or bilirubin samples should be stored in a dark bag, even in the refrigerator.
Storage specifics for serum and plasma
After centrifugation, the blood is separated into a dense sediment (cells) and the liquid part. The liquid portion may be plasma (if an anticoagulant has been added) or serum (if the blood has clotted). It is these fractions that are most often sent for long-term storage or transportation.
Serum is more stable than whole blood. There are no cells in it that could continue metabolism. However, the risk of bacterial contamination remains. If the tube has been opened or is not sterile, bacteria will begin to multiply even at +4°C, consuming glucose and releasing ammonia.
⚠️ Caution: Never freeze tubes of whole blood or serum in a regular freezer unless specifically instructed to do so by your physician. The formation of ice crystals ruptures cell walls and protein structures.
For most hormonal studies, serum can be stored in the refrigerator for up to 3 days. If the result is needed later, it must be frozen at a temperature of -20°C or lower. Repeated freezing and thawing is strictly prohibited.
Features of storage of different types of tests
Different areas of medicine require different approaches. In hematology, the most important thing is to preserve the integrity of cells, and in biochemistry, the chemical composition of dissolved substances.
Let's consider the main groups:
- 🩸 Hematology: Tubes with EDTA (purple) are best stored at room temperature. Cold can cause platelet aggregation, and the analyzer will count false thrombocytopenia.
- 🧪 Biochemistry: Serum can be stored at +4°C for up to 48 hours. The exception is tests for renin and angiotensin, which require immediate cooling and freezing.
- 🦠 Microbiology: Blood cultures (bakposev) are usually not stored for a long time. They need to be delivered in a thermostat at a temperature of 35-37°C, since bacteria stop growing in the cold, and the analysis will be false negative.
Special attention is required coagulograms (coagulability tests). Plasma for them (blue tubes) cannot be stored for more than 2-4 hours, even in the refrigerator, since coagulation factors are quickly destroyed.
☑️ Checking the sample before delivery
Transportation: how to deliver blood to the laboratory
Often a situation arises when blood needs to be donated at a remote point or brought from home (for example, daily urine or rare tests). The main enemies during transportation are shaking, temperature changes and direct sunlight.
Use special thermal containers or cold packs. However, remember: the cold element should not touch the test tube directly! This can lead to local freezing of the sample near the wall of the tube. Always use a layer of fabric or a stand.
During transportation, the test tube should be kept vertically so that the lid does not leak. Sudden movements of the car can cause mechanical hemolysis. If you are bringing blood in winter, do not leave it in the car for a long time, even if it is frosty outside - it may be warm in the cabin, and the sample will become hypothermic near the window.
⚠️ Attention: Storage conditions may vary depending on the reagents of a particular laboratory. Always check the requirements with the registry before transporting biomaterial yourself.
Frequent errors during storage of biomaterial
Errors at the preanalytical stage account for up to 70% of all laboratory errors. Patients often underestimate the importance of proper storage.
The most common mistake is storing the tube next to food. Vapors from alcohol, spices, or chemicals (such as vinegar or cleaning products) can penetrate rubber stoppers and interfere with gas composition or toxicology results.
- ❄️ Hypothermia: Place the tube at the back of the refrigerator, where the temperature may drop below 0°C.
- ☀️ Light: Storage on refrigerator door or on the table under a bright lamp (destruction of B vitamins and bilirubin).
- ⏳ Time: Delay of delivery by more than 24 hours without freezing the serum.
Another error — an attempt to “warm” frozen blood in a microwave or hot water. This is guaranteed to denature the proteins and render the sample unusable. Thawing can only be done slowly, at room temperature or in a special thermostat.
Is it possible to donate blood that has stood in the refrigerator overnight?
For a general blood test (CBC), it is usually possible if no more than 12-24 hours have passed and the tube has been tightly closed. For biochemistry and hormones - absolutely not, unless the serum was separated from the cells immediately after collection. Whole blood will change its properties overnight in the refrigerator.
What to do if the blood in the test tube has coagulated, although there was an anticoagulant there?
Such a sample is unsuitable for analysis. Coagulation indicates that the blood was not sufficiently mixed immediately after collection or the anticoagulant was of poor quality. There is no point in taking such an analysis; the result will be false. You need to retake it.
Does the fact that the test tube was lying horizontally affect the analysis?
Yes, it can. In a horizontal position, the area of contact of blood with the plug and air increases (if there is a microgap). This can lead to evaporation of moisture and changes in the concentration of substances, as well as oxidation. It is better to store vertically.
How long is blood good for after defrosting?
After defrosting, serum or plasma should be used within 2-4 hours. It cannot be re-frozen. If you defrosted the sample, but did not have time to hand it over, it will have to be disposed of.