Donating blood for analysis is a routine but critically important procedure, on the accuracy of which diagnoses and treatment depend. However, few people think about what happens to the test tube after the material is collected: how long it can be stored in the refrigerator, why some tests require urgent delivery to the laboratory, while others allow storage for up to 48 hours. Errors at this stage lead to false-positive or false-negative resultsrepeated tests and even missed diagnoses.
In this article we will examine standard storage periods for blood in test tubes with different anticoagulants (EDTA, citrate, heparin) and without them, the effect of refrigerator temperature (+2...+8°C) on the stability of samples, as well as typical mistakes that healthcare workers and patients make during independent transportation. We will pay special attention critical tests (glucose, D-dimer, blood gases), where even a 30-minute delay distorts the result.
Why blood storage periods depend on the type of tube
Tubes for blood collection differ in two key parameters: presence of an anticoagulant and its type. Anticoagulants prevent clotting, but each has time limits on cell stability and biochemical markers. For example:
- 🩸 EDTA (purple cap) - used for a general blood test (CBC), but when stored for >24 hours, red blood cells begin lysis (destruction), which distorts hemoglobin and hematocrit indicators.
- 💙 Sodium citrate (blue cap) — optimal for a coagulogram, but if the blood/citrate ratio (9:1) is violated, the results of tests for INR and APTT become unreliable after 4 hours.
- 🟢 Heparin (green cap) —suitable for biochemical tests, but unstable for glucose: its level drops by 5–7% every 2 hours due to glycolysis.
- 🟤 Without anticoagulant (red/yellow cap) —blood clots in 30–60 minutes, so such tubes are used only for serum (for example, for hormones) and stored for no longer than 8 hours.
In addition to the anticoagulant, the timing is affected tube material: glass or plastic. Plastic tubes (for example Vacutainer or BD Microtainer) reduce the risk of hemolysis, but do not increase shelf life - they only minimize losses during transportation.
Table: shelf life of blood refrigerator (+2...+8°C) by type of analysis
| Type of analysis | Color of the tube cap | Anticoagulant | Max. shelf life | Notes |
|---|---|---|---|---|
| General blood test (CBC) | Purple | EDTA | 24 hours | After 12 hours, hemolysis of red blood cells is possible |
| Biochemistry (glucose, electrolytes) | Green/gray | Heparin/fluoride | 4–8 hours | Glucose decreases by 0.5 mmol/l each hour |
| Coagulogram (INR, APTT) | Blue | Sodium citrate | 4 hours | At 20°C - no more than 2 hours |
| Blood gases (pH, pO₂, pCO₂) | Grey | Heparin (sealed) | 30 minutes | Store only in ice water (+4°C) |
| Hormones (TSH, cortisol) | Red/yellow | Without anticoagulant (serum) | 48 hours | After centrifugation - up to 7 days at −20°C |
⚠️ Attention: the data in the table is relevant for non-centrifuged blood. After separating serum or plasma (centrifugation), shelf life increases by 2–3 times. For example, serum for hormone analysis can be stored in the refrigerator for up to 72 hours, and in the freezer (−20°C) for up to 1 month.
How does refrigerator temperature affect sample stability
The optimal range for storing blood tubes is +2…+8°C. Deviations in any direction accelerate the degradation of the sample:
- ❄️ Below +2°C: the risk of crystallization of water in the plasma, which leads to a false increase in the level of potassium and lactate dehydrogenase (LDH).
- 🔥 Above +8°C: glycolysis (breakdown of glucose) is accelerated, which distorts the results of biochemistry. For example, at +25°C, glucose decreases by 10% in 1 hour.
- 🌡️ Temperature changes: condensation on the tube dilutes the sample, reducing the concentration of analytes (for example, hemoglobin).
For critical analyzes (blood gases, ammonia) use ice baths (+0...+4°C) or special containers with cooling elements. It is difficult to maintain this temperature in household refrigerators - it is better to use thermal containers with cold accumulators.
Typical errors when storing blood in the refrigerator
Even if the storage period is observed, errors at the stage of preparation or placement of the tube nullify all efforts. Common mistakes:
- Incorrect blood/anticoagulant ratio. For example, in a test tube with citrate there should be strictly 9 parts of blood to 1 part of citrate. If this balance is disturbed, the results of the coagulogram will be falsely high or low.
- Storage in refrigerator door. Here the temperature fluctuates by 3-5°C with each opening, which accelerates the degradation of glucose and enzymes.
- The use of a household refrigerator for blood gases. Even at +4°C pO₂ and pCO₂ change by 5–10% in 30 minutes. Such analyzes require analyzers at the patient's bedside (POCT).
- Lack labeling. Test tubes without indicating the time of collection or type of analysis are often confused, which leads to repeated tests.
⚠️ Attention: if the test tube was removed from the refrigerator for transportation, it cannot be returned. Temperature changes trigger irreversible biochemical reactions. For example, in a test tube with EDTA, after thawing, red blood cells are lysed in 1–2 hours.
☑️ Check before storing the test tube
What happens to blood if the storage period is violated
Exceeding the recommended storage periods leads to biochemical and cellular changeswhich distort the results:
- 🔬 Hemolysis: The destruction of red blood cells increases the level of free hemoglobin, potassium and LDH. Falsely inflates the indicators for CBC and biochemistry.
- 🧬 Glycolysis: in test tubes without sodium fluoride, glucose is consumed by cells at a rate of 0.5–1 mmol/l per hour. After 6 hours, the result may be underestimated by 30%.
- 🦠 Bacterial contamination: When stored for >24 hours in non-sterile conditions, the growth of microorganisms is possible, which distorts the ESR and leukocyte formula.
- ⚗️ Cagulation: in test tubes without an anticoagulant, microclots form after 1–2 hours, which clog the analyzer channels, making analysis impossible.
For example, during analysis on D-dimer (marker of thrombosis), storing a tube with citrate for >4 hours gives a false negative result in 30% of cases. This is dangerous when diagnosing pulmonary embolism or disseminated intravascular coagulation syndrome.
Case Study
After 12 hours of storage of a tube with EDTA for CBC in a patient with leukemia, the level of leukocytes was underestimated by 40% due to cell destruction. This led to a delay in the start of chemotherapy by 3 days.
How to properly transport tubes from the refrigerator to the laboratory
If the blood was stored in the refrigerator, its transportation to the laboratory should take a minimum of time. Optimal conditions:
- ⏱️ Time: no more than 1 hour at room temperature (for biochemistry) or 30 minutes for coagulogram.
- 🧊 Temperature mode: use thermal containers with cold accumulators (+2...+8°C). For blood gases - ice bath (+0...+4°C).
- 🚗 Tube position: vertical (lid up) to avoid mixing serum with cells.
- 📝 Documentation: attach direction indicating time of collection, type of tube and storage conditions.
⚠️ Attention: if the tube was frozen (for example, for analysis of hormones), it must be thawed at +4°C in the refrigerator, and not at room temperature. Rapid thawing destroys protein structures, distorting the results of enzyme-linked immunosorbent tests (ELISA).
FAQ: Frequently asked questions about storing blood in test tubes
Can a tube of blood be stored in the freezer?
Freezing is allowed only for serum or plasma (after centrifugation) at −20°C or −80°C. Whole blood is destroyed when frozen: red blood cells are lysed and proteins are denatured. An exception is special studies (for example, cryopreservation of stem cells) where cryoprotectants are used.
What to do if the tube was stored longer than the permissible period?
Such a sample is considered unsuitable for analysis. Repeated blood sampling is required, as the results will be unreliable. An exception is some hormonal tests (for example, cortisol), where serum storage for up to 7 days at −20°C is allowed, but this must be agreed with the laboratory.
Can blood be transfused from one tube to another for storage?
No. Transfusion disrupts the blood/anticoagulant ratio and increases the risk of hemolysis and bacterial contamination. If the tube is damaged, it is better to take a second sample. The exception is transfusion into sterile Eppendorff for long-term storage of serum (only in laboratory conditions).
How to check whether the blood in a test tube has spoiled?
Signs of unfitness sample:
- 🔴 Hemolysis: plasma/serum pink or red.
- 🟤 Clots: visible clots or turbidity (for tubes without anticoagulant).
- 🦠 Contamination: flakes, foam or unpleasant odor (bacterial growth).
- ❄️ Crystals: white flakes in frozen whey (protein denaturation).
For any of these signs, analysis repeat.
Does the type of refrigerator (No Frost, drip) affect the storage of test tubes?
The type of defrosting is not critical if the temperature is stably kept in the range of +2...+8°C. However, in refrigerators No Frost the air is drier, which can lead to evaporation of liquid from the test tube through microcracks in the lid. For long-term storage (>24 hours), it is better to use refrigerators with dynamic cooling and the "super cooling" function.