Question about the permissible storage time of biological material before laboratory testing often occurs in patients, especially when the analysis takes place in the evening or on weekends. Sample freshness is a critical factor for obtaining a reliable result, since many microorganisms and chemical compounds in feces are unstable at room temperature. Errors at the preanalytical stage, which include improper storage, can lead to false positive or false negative conclusions, which will require repeating the procedure.
In most cases refrigerator compartment becomes the only available way to slow down the processes of decomposition and proliferation of bacteria. However, there is no universal answer for all types of diagnostics: the requirements for transportation conditions and timing depend on the purpose of the study. Some tests require immediate delivery, others allow storage for 24 hours, and still others do not tolerate freezing at all. Understanding these nuances will help you avoid wasting time and money.
In this article we will analyze in detail how long and under what conditions biomaterial can be stored so that the laboratory accepts it without complaints. We will consider the influence of temperature on various biomaterial parameters and give clear recommendations on the use of special containers. Compliance with collection rules is the first step towards accurate diagnosis and effective treatment.
The influence of temperature on the quality of biomaterial
Temperature plays a decisive role in preserving the properties of feces. At room temperature, the processes of fermentation and decay actively continue in the sample, and the microflora rapidly multiplies. Bacterial composition changes literally every minute, which makes it impossible to accurately determine pathogenic organisms or dysbacteriosis. That is why refrigeration is a prerequisite for most types of analysis.
The optimal temperature for short-term storage is considered to be the range from +2 to +8 degrees Celsius. This is the standard mode of operation of the main camera of a household refrigerator. It is important to understand that we are talking specifically about the shelf, and not about the door, where the temperature fluctuates more strongly, and certainly not about the freezer. Freezing destroys the cell walls of many bacteria and changes the structure of helminth eggs, making them invisible to the microscope.
It is worth noting that even in the cold, biological processes do not stop completely, but only slow down. Therefore, the concept of “storage” in this context is relative: it is only a way to gain time to deliver the sample to the laboratory, but not to preserve it forever. Long stay in the refrigerator can also distort the results, especially when it comes to latent blood or specific enzymes.
General rules for collection and preparation containers
The quality of the analysis directly depends on how correctly the material was collected. For collection, you must use only sterile disposable containers, which are sold in pharmacies. The use of household containers, medicine jars or matchboxes is strictly prohibited, as they may contain impurities that affect the sample. chemical composition sample.
Before defecation, it is necessary to prepare a clean surface. Avoid collecting material from the toilet where it may become mixed with water or disinfectants. It is best to use a clean, dry pot, vessel, or plastic wrap over a toilet seat. The collection is carried out with a special spoon built into the lid of the container from 3-4 different places of feces.
☑️ Rules for collecting biomaterial
The volume of collected material also matters. For most studies, an amount equal to one to two teaspoons (about 5-10 grams) is sufficient. Liquid stool is collected with a pipette or syringe without a needle, filling the container about a third. After collection, the container must be tightly closed to avoid drying out and the entry of foreign odors, which may affect organoleptic characteristics. The answer to this strictly depends on what exactly the laboratory technician is looking for. Different research methods impose their own requirements on organoleptic indicators.
Shelf life for various types of analyzes
The most common question from patients is: “How many hours can the test be kept in the cold?” The answer to this strictly depends on what exactly the laboratory technician is looking for. Different research methods have their own requirements for sample freshness. Below is a table systematizing data on the main types of diagnostics.
| Type of research | Maximum storage period | Optimal conditions | Acceptability freezing |
|---|---|---|---|
| Eggworm test | Up to 24 hours | +2..+8°C | Not recommended |
| Occult blood test | Up to 12 hours | +2..+6°C | Absolutely not |
| Bacteriological culture | Up to 3-4 hours | Room t° or special. Wednesday | No |
| Analysis for dysbacteriosis | Up to 2-3 hours | Thermal container with ice | No |
As can be seen from the table, bacteriological studies are the most demanding. To sow flora, the material must be delivered to the laboratory as quickly as possible, preferably within an hour. If this is not possible, some laboratories allow the use of special transport media (for example, Cary-Blair), which stabilize the microflora, but you need to find out about this in advance.
Why can’t you freeze feces?
Freezing leads to the formation of crystals ice, which breaks the membranes of bacteria and parasite eggs. When defrosted, the structure of the sample irreversibly changes, which makes microscopic examination and cultivation of bacteria impossible.
For analysis occult blood not only time is important, but also the absence of oxidation. Long-term storage, even in the refrigerator, can lead to the destruction of hemoglobin under the action of enzymes contained in the stool itself, which will give a false negative result. Therefore, here the rule “the faster, the better” works without exception.
Features of storing an analysis for dysbacteriosis
Diagnostics of dysbiosis requires a special approach, since the purpose of the study is to determine the exact ratio of different types of bacteria in the intestine. Microflora outside their usual habitat begins to change very quickly: some bacteria die, others, on the contrary, begin to multiply uncontrollably. Aerobic conditions (contact with oxygen) are destructive for many anaerobes living in the intestines.
Ideally, the material for this analysis should be delivered to the laboratory immediately after defecation, warm. However, if this is not possible, short-term storage in the refrigerator is allowed, but no more than 2-3 hours. Some modern laboratories offer special preservatives that can extend the life of a sample to up to a day, but they can only be used as prescribed by a doctor.
These drugs must be discontinued a few days before collecting the material, if the patient’s health condition allows it. Stool consistency It is also important: a decorated stool is more informative for assessing dysbiosis than liquid.
The nuances of the analysis for occult blood
A fecal occult blood test (Gregersen reaction or immunochemical test) is aimed at identifying microscopic inclusions of hemoglobin that are not visible to the eye. This is a key method for early diagnosis of colorectal cancer and polyps. However, hemoglobin is an unstable substance, and under the influence of digestive enzymes it is quickly destroyed.
If you collected the material in the evening and plan to hand it over in the morning, storage in the refrigerator at a temperature of +4 degrees is mandatory. The maximum period in this case should not exceed 10-12 hours. Longer storage will cause the red blood cells to be destroyed, and the test will show a negative result even if there is bleeding.
⚠️ Attention: Before taking the occult blood test, you must follow a diet. 3 days before the test, exclude meat, fish, green vegetables (cucumbers, cabbage, broccoli), apples and iron supplements from your diet. This is necessary to exclude false-positive results when using chemical methods.
Modern immunochemical tests are less dependent on diet, but the requirements for the freshness of the material remain strict. Container must be clean and dry, without traces of detergents that may react with the reagents test.
Errors during storage and transportation
Even knowing the theoretical deadlines, patients often make practical mistakes that nullify all efforts. One of the most common is storing the container on the refrigerator door. There, the temperature is constantly changing due to the opening of the door, which creates stressful conditions for the biomaterial. In addition, there may be products nearby that have a strong odor, which can be absorbed into the plastic.
Another mistake is using unsealed containers. If the lid is not closed tightly, the feces may dry out or, conversely, become saturated with moisture from the refrigerator compartment. This also changes the concentration of substances, making the analysis uninformative. Also, you should not place the container next to frozen products, since local hypothermia can damage the structure of the sample. consistency and concentration of substances, making the analysis uninformative. Also, you should not place the container next to frozen foods, as local hypothermia can damage the structure of the sample.
Transportation as well requires attention. In winter, the material cannot be carried in your hands without protection - it can become hypothermic. In summer, on the contrary, in the heat, it is better to place the container in a thermal bag or wrap it with a cold element so that the temperature does not rise above the permissible level. Thermal stability is the key to successful delivery.
Frequently asked questions (FAQ)
Is it possible to collect a stool test in the evening and submit it in the morning?
Yes, this is possible for most general tests (for eggworm, coprogram), if you store the container in the refrigerator at a temperature of +2..+8°C. However, the storage period should not exceed 8-10 hours. For tests for dysbacteriosis and occult blood, evening collection is undesirable, since the results may be distorted.
What to do if stool is frozen in the refrigerator?
Such material cannot be used for analysis. Freezing destroys the cellular structures of bacteria, helminth eggs and protozoa. Microscopic examination will show an invalid picture, and bacteriological culture will not give growth of the culture. The material will have to be collected again.
Is it necessary to warm up the feces before donation if it was in the refrigerator?
No, specially heating the material (on a radiator, in a microwave, in hot water) is strictly prohibited. A sharp change in temperature will destroy beneficial microflora and change the chemical composition. The material is submitted in the form in which it was stored (chilled), or brought to room temperature naturally for 30-40 minutes before delivery, if time permits.
Is it possible to store a stool test in the freezer?
No, biomaterial for standard clinical tests cannot be stored in the freezer. Low temperatures (-18°C and below) irreversibly change the structure of the sample. The exception is special studies conducted for scientific purposes, where the material is immediately frozen at ultra-low temperatures using liquid nitrogen, but this is done in a laboratory.