Terms and conditions for storing blood in the refrigerator for analysis

The question of how long blood can be stored in the refrigerator is critical to obtaining reliable laboratory results. Errors at the preanalytical stage, which includes transportation and storage of biomaterial, account for up to 70% of all diagnostic errors. Hemolysis, a change in temperature or waiting too long for an analysis can completely distort the picture of the patient's condition, forcing the doctor to prescribe the wrong treatment or miss a dangerous pathology.

For most standard tests, such as a complete blood count or biochemistry, there are strictly regulated time windows. Usually we are talking about several hours at room temperature, but when placing the test tube in the refrigerator, these periods may vary depending on the type of study. EDTA, sodium citrate or clotting activators - the anticoagulants used determine how long the cells will retain their structure and chemical composition.

In this article we will look at how Properly organize the storage of samples to avoid retaking. You will learn why some tubes cannot be refrigerated, while others require immediate refrigeration, and what factors can destroy the sample most quickly.

Factors affecting the stability of a biomaterial

The safety of a biological sample depends on many variables. The main enemy here is time. The longer the blood is outside the body, the more active metabolic processes occur in it, which change its chemical composition. Glycolysis —the process of breaking down glucose—continues even after blood sampling, which leads to a false decrease in sugar levels and an increase in lactate levels. That is why the time interval between collection and centrifugation or analysis should be minimal.

The second critical factor is the temperature regime. The refrigerator compartment, as a rule, maintains a temperature of +2 to +8 degrees Celsius. For some blood components, such as some hormones or enzymes, cold is necessary to “freeze” chemical reactions. For others, such as cryoglobulins or clotting factors, sudden cooling can be fatal, causing precipitation or platelet activation.

⚠️ Warning: Never freeze whole blood in a household refrigerator! The formation of ice crystals destroys the cell membranes of red blood cells, causing massive hemolysis, making the sample unsuitable for most tests.

It is also important to consider the type of tube and the reagents added to it. Vacuum systems contain strictly dosed amounts of anticoagulants. Violation of the “blood-additive” proportion or improper mixing can lead to the formation of microclots that will clog the analyzer or distort the cell count.

General analysis blood: features of storage with EDTA

A complete blood count (CBC) is the most common test. It uses a test tube with a purple cap containing EDTA (ethylenediaminetetraacetic acid). This anticoagulant binds calcium, preventing clotting, and preserves cell morphology. However, EDTA does not stop all biochemical processes.

At room temperature, the morphology of leukocytes and platelets begins to change after 2-3 hours. Platelets may swell or, conversely, fragment, leading to an erroneous diagnosis of thrombocytopenia or thrombocytosis. If the analysis cannot be carried out within 2-3 hours after collection, it is recommended to place the sample in the refrigerator. In cold conditions (+4..+6°C), the stability of the cellular composition is maintained for up to 24 hours, although a period of up to 12 hours is considered optimal.

• 🩸 When stored in the refrigerator, cell metabolism slows down, which allows them to maintain their size and shape longer.
• ⏳ After 24 hours, even in the cold, degradation of granulocytes begins, and the results of the leukocyte count formulas become unreliable.
• 🌡️ Sudden temperature changes (taken out of the refrigerator, kept in the light, removed again) are strictly prohibited.

This is necessary to equalize the temperature and prevent condensation inside the device.

Biochemical analysis: serum and temperature

For biochemical studies, blood without anticoagulants (test tubes with a red or yellow cap) is most often used, in which the serum coagulates and is released. There are rules here. After centrifugation, the serum can be stored in the refrigerator at a temperature of +2..+8°C. Storage periods depend on the parameter being studied.

Some enzymes, such as LDH (lactate dehydrogenase) and CK (creatine kinase), are very unstable. Their activity may decline after just a few hours. Serum glucose also decreases faster than in whole blood with sodium fluoride, so it is better to take biochemistry tests in the morning on an empty stomach and deliver them to the laboratory as quickly as possible. If the analysis is not carried out immediately, the serum must be separated from the cell mass (sediment) as soon as possible.

Algorithm of actions when the analysis is delayed:
1. Let the test tube stand for 30-60 minutes to clot.
2. Centrifuge for 10-15 minutes at 1500-2000 rpm.
3. Carefully separate the serum into a clean test tube (with a pipette).
4. Place the serum in the refrigerator (+4°C).

Contact with cell mass (erythrocytes) is detrimental to many indicators. Potassium, magnesium, and enzyme ions can escape from the cells into the serum, which will falsely increase their concentration. Therefore, the “separate and save” rule works flawlessly here.

Coagulology: why citrate does not like frost

The study of the hemostatic system (coagulogram) requires blue-capped tubes containing sodium citrate. This is one of the most capricious types of analysis. Clotting factors are extremely sensitive to storage conditions. The standard shelf life of whole citrated blood for research is no more than 4 hours at room temperature.

Cooling such blood often leads to activation of Hageman factor (XII) and a change in the time of activated partial thromboplastin time (aPTT). Therefore, unlike the OAC, coagulological samples not recommended are stored in the refrigerator, unless otherwise indicated in the instructions for a specific test. Usually the laboratory requires delivery of the material within 1-2 hours.

⚠️ Attention: Do not shake the test tube with citrate! This will cause platelet activation and distort the results. Mixing should be gentle, by inverting the tube 3-4 times immediately after collection.

If the test is delayed for more than 4 hours, it is necessary to centrifuge the sample and freeze the plasma at -20°C or lower. But this is the task of laboratory specialists, and not the patient or nursing staff at the collection point.

Hormonal studies and tumor markers

Hormonal background is a dynamic value. Many hormones are destroyed by enzymes present in the blood. Strict time limits apply for studies of thyroid hormones, sex hormones, cortisol and tumor markers. Typically, serum can be stored in the refrigerator for up to 2-3 days if the analysis is not carried out on the day of collection.

However, there is a nuance with ACTH (adrenocorticotropic hormone) and insulin. These molecules are very unstable. Blood for ACTH testing often needs to be transported to the laboratory on ice and spun in a refrigerated centrifuge. Insulin also degrades quickly. For such specific tests, storage time in the refrigerator can be reduced to several hours.

If long-term storage of samples for hormonal studies is planned (more than 3 days), the serum must be frozen at -20°C. It is important to avoid freeze-thaw cycles, since each such cycle destroys part of the protein molecules.

Why can’t you store blood in the refrigerator door?

The door is the warmest place in the refrigerator, and the temperature there often rises above +8°C due to frequent opening. In addition, vibrations are possible there. To preserve the biomaterial, use the middle shelves, away from the walls, to avoid local freezing.

Typical mistakes when storing test tubes at home

Often patients or nurses at home make mistakes when trying to “save” the blood until the courier arrives or transports it to the laboratory. One of the main mistakes is placing the test tube near food. This is not only unhygienic, but also risky: the products can release volatile substances or become a source of bacterial contamination if the seal of the tube is broken.

Light is another common problem. Direct sunlight or bright light from a lamp can destroy some sensitive components, such as bilirubin. Bilirubin levels in blood exposed to light can drop by 50% in an hour. Therefore, tubes should always be protected from light; it is best to keep them in an opaque container or box.

• ❌ Do not place tubes on top of the freezer or close to the back wall of the refrigerator - local freezing is possible there.
• ❌ Do not store tubes horizontally, unless they are intended for this purpose - this increases the area of contact of blood with the lid and a stopper.
• ❌ Do not label test tubes with markers that are erased from condensation - use labels or indelible markers.

It is also worth mentioning the “tourniquet effect”. If blood was taken with a long tourniquet applied, hemolysis or changes in protein concentrations could begin in it already at the time of collection. Such blood will spoil in the refrigerator faster than that taken according to the rules.

Transportation: how to transport the sample correctly

Once you have decided With shelf life, the question of transportation arises. If you are carrying the analysis yourself or it is being carried by a courier, it is important to use special thermal containers. A regular bag of l