Timely delivery of biological material to the laboratory is a critical factor determining the reliability of the results of a medical study. Many patients mistakenly believe that the collected sample can be left on the refrigerator shelf indefinitely, but biological processes continue even at low temperatures. The enzymatic activity and vital activity of bacteria can change the chemical composition of the intestinal contents, which will lead to a false positive or false negative result.
Proper organization of the process of collecting and storing biomaterial requires strict adherence to temperature conditions and time intervals. If you plan to take the test in the morning, and the collection took place in the evening, it is necessary to create conditions that slow down the decomposition of the components as much as possible. In this article we will analyze in detail how long a material retains its properties and what factors can accelerate its deterioration.
The influence of time on the quality of a biomaterial
Biological samples intended for diagnostics are subject to rapid degradation under the influence of the external environment. The main enemy here is time, since with every minute after defecation, irreversible changes begin in the mass. Enzymescontained in digestive juices continue to break down proteins and other organic compounds, which can distort the picture of the presence of hidden blood.
Laboratory studies show that after 3-4 hours of being at room temperature, the composition of the sample changes dramatically. Bacteria begin to actively multiply, releasing waste products that react with the reagents of the test systems. That is why the gold standard is considered to be delivery of the material to the laboratory within 1.5–2 hours after collection.
If it is impossible to maintain this time period, the refrigerated storage rule comes into force, but it also has its own strict limitations. Prolonged exposure to cold also does not guarantee the safety of all parameters, especially when it comes to sensitive methods for determining hemoglobin. It is important to understand that even in the cold, oxidation processes do not stop completely, but only slow down.
Optimal temperature conditions for storage
Temperature conditions are the second most important factor after storage time, affecting the suitability of the sample for research. The standard chamber of a household refrigerator, where the temperature fluctuates within +2...+4°C, is suitable for short-term preservation of biomaterial. However, placing the container on the door shelf or at the back wall, where freezing is possible, is strictly prohibited.
⚠️ Attention: Freezing stool for occult blood testing is unacceptable! Ice crystals destroy the cellular structures of red blood cells, making it impossible to detect hemoglobin, which will lead to a false negative result.
It is necessary to ensure temperature stability, avoiding sudden changes. If you take a container out of the refrigerator to take it to the clinic, you should not return the leftovers for re-delivery the next day. Re-cooling material that has already been in the heat will trigger accelerated rotting processes.
The ideal placement location is the middle shelf of the refrigerator, away from freezer and heat sources. The container must be tightly closed to prevent the entry of foreign odors or microorganisms from the outside, although the main risk comes from internal processes. Compliance with temperature discipline is your guarantee that the doctor will receive an objective picture of your health.
How long can feces last in the fridge
The answer to the question about the permissible storage time depends depending on the type of research performed and the reagents used. For the classic test for occult blood (Gregersen reaction), the maximum shelf life in the refrigerator is 12 hours. Exceeding this limit significantly reduces the information content of the sample, since hemoglobin is destroyed by bacteria and enzymes.
Modern immunochemical methods (iFOBT) are more sensitive and specific, but their requirements for the freshness of the material are even stricter. In some cases, laboratory technicians recommend not storing such material at all, but handing it over immediately after collection. If the instructions for a specific test system allow storage, then usually we are talking about a period of no more than 6–8 hours.
Below is a table showing the dependence of the quality of the analysis on storage time under various conditions:
| Storage conditions | Temperature | Maximum period | Risk of distortion |
|---|---|---|---|
| Room temperature | +20...+23°C | up to 2 hours | High |
| Refrigerator compartment | +2...+4°C | up to 12 hours | Average |
| Frozen | -18°C and below | Unacceptable | Critical |
| C preservative | +2...+8°C | up to 3 days | Low |
It is worth noting that the indicated periods are relevant for correctly collected material in a sterile container. If water, urine or hygiene products get into the sample, its shelf life tends to zero, regardless of temperature. Water dilutes the masses and changes the osmotic pressure, causing lysis (destruction) of red blood cells.
Use of preservatives
In cases where delivery of material to the laboratory in the morning is impossible, or it is necessary to collect material in the evening of the previous day, special preservatives can be used. Such substances are added to the container before collection or they are already contained in the lid of special collection systems. Preservatives stabilize the structure of the biomaterial and prevent the growth of bacteria.
The most common preservative is glycerin or special buffer solutions. When using such systems, the shelf life of stool in the refrigerator can be increased to 3 days. However, this method can only be used with the agreement of a doctor or if the laboratory has given you the appropriate container with instructions.
It is important not to confuse preservatives for microbiological studies and for occult blood tests. In the first case, the goal is to preserve living cultures of bacteria, in the second, to prevent the breakdown of hemoglobin. Using the wrong fixative can completely destroy the markers you are looking for.
⚠️ Attention: Never add foreign substances (alcohol, vinegar, antibiotics) to the analysis container yourself. This is guaranteed to spoil the sample and make the study meaningless.
What to do if you forgot to add a preservative?
If the system included the use of a preservative, and you forgot to add it or mix it with the sample, the shelf life is reduced to the standard 12 hours. In this case, it is better to retake the analysis than to get a false result.
Rules for collecting material for an accurate result
The quality of the analysis depends not only on the storage time, but also on the collection technique. To test for occult blood, it is necessary to follow a diet and eliminate factors that may affect the result 2-3 days before the procedure. The process of defecation itself should occur naturally, without the use of enemas or laxative suppositories.
- 🚽 Use a clean, dry vessel or stretch cling film over the toilet seat so that the feces do not come into contact with water and disinfectants.
- 🥄 Take the material with a special with a spoon built into the lid of the container, taking small portions from 3-4 different places of the fecal mass.
- 🧼 Wash thoroughly before defecation to prevent blood from the genitourinary tract or menstrual flow from entering the sample.
The volume of collected material should be approximately equal to the volume of a teaspoon (5-10 grams). An amount that is too small may not be sufficient to allow re-testing if the result is questionable. The container must be hermetically sealed immediately after collection.
Particular attention should be paid to hand hygiene and the collection container. Even minimal contamination by foreign particles can cause errors in the operation of automatic analyzers. If you use household utensils instead of a sterile container, they should be perfectly washed and dried, although the use of medicine cups is preferable.
☑️ Checklist for preparing for analysis collection
Factors distorting analysis results
There are a number of factors that can lead to false results, even if the shelf life has been met. First of all, this concerns diet. Traditional chemical methods (Gregersen reaction) react on hemoglobin of both humans and animals. Therefore, eating meat products, liver or blood sausage may give a false positive result.
Some medications also affect the result. Aspirin, non-steroidal anti-inflammatory drugs (NSAIDs) and iron supplements can change the color of stool or provoke microbleeding in the gastrointestinal tract. You must inform your doctor about taking any medications before prescribing the test.
Menstruation in women is an absolute contraindication for taking the test during this period. Blood from the genital tract will inevitably end up in the sample, making diagnosis impossible. It is recommended to wait 2-3 days after the end of menstruation.
⚠️ Attention: If you have undergone endoscopic examinations (colonoscopy, gastroscopy) or X-ray with barium, you can take an occult blood test no earlier than 3 days after the procedure.
Frequently asked questions (FAQ)
Is it possible to store stool for analysis in the freezer if you need to keep it for longer than 12 hours?
No, you absolutely cannot freeze stool for occult blood testing. When frozen, the cell walls of red blood cells break down and hemoglobin is released and destroyed, causing a false negative result. If you do not have time to deliver the material within 12 hours, it is better to retake the analysis on another day.
Does the tightness of the container lid affect the shelf life?
Yes, the tightness of the container is critically important. A loose lid will cause the sample to dry out, oxidize the components, and allow foreign odors and bacteria to escape from the refrigerator. This will accelerate the deterioration of the material and make it unsuitable for research long before the expiration of 12 hours.
What to do if feces were collected in the evening, but in the morning it was not in the refrigerator (someone threw it away)?
In this case, there is no point in handing over material that has been in the refrigerator for more than 12 hours (or an unknown time) - the result will be incorrect. It is necessary to collect a new portion. If morning collection is not possible, try collecting the material in the daytime and delivering it to the laboratory within 2 hours, having previously clarified the laboratory's work schedule.
Can a regular plastic bag be used for storage instead of a container?
The use of regular plastic bags is not recommended, since they are not sterile and may contain chemical additives that react with biomaterial. In addition, the bag is difficult to seal tightly without the risk of leakage. It is better to use special pharmacy containers with a spoon, which are inexpensive and guarantee the safety of the sample.