Submitting stool for analysis is a routine but responsible procedure, the accuracy of which determines the accuracy of the diagnosis. One of the key questions: How long can the material last in the fridgeif it is not possible to deliver it to the laboratory immediately after collection. Errors at this stage lead to false-positive or false-negative results, which is especially critical when examining for parasites, occult blood or dysbacteriosis.
In this article we will look at stool storage standards for different types of tests (coprogram, PCR, worm eggs, etc.), optimal temperature conditions, as well what absolutely cannot be done during the preparation of the material. Let us separately dwell on the nuances for children, the elderly and patients with chronic gastrointestinal diseases - for them the rules may differ.
Why is it important to observe the storage periods for feces?
Feces are biologically active an environment in which enzymatic processes, proliferation of microorganisms and cell breakdown continue after defecation. At room temperature, these processes accelerate significantly, which leads to:
- 🔬 Change in pH —in 3–4 hours at +20°C, acidity can shift by 1–2 units, which will distort the results of the coprogram.
- 🦠 Reproduction of pathogenic bacteria —for example, Salmonella or E. coli can “clog” the growth of other microorganisms important for analysis for dysbacteriosis.
- 🩸 Decomposition of red blood cells - when searching for occult blood (hemoglobin test), a false negative result is guaranteed after 6 hours.
- 🐛 Death of parasites -helminth eggs (for example, roundworms) lose viability when drying out or temperature changes.
The refrigerator slows down these processes, but does not stop completely. Even at +4°C pinworm eggs remain invasive for up to 72 hours, and vegetative forms of Giardia - no more than 24 hours. Therefore, storage periods are strictly regulated for each type of analysis.
Optimal storage time for feces in the refrigerator
General rule: the faster the material gets to the laboratory, the more accurate the result will be. However, in reality, it is not always possible to deliver feces immediately. Below - maximum permissible shelf life for different studies at temperature +2…+8°C (standard refrigerator mode):
| Type of analysis | Maximum shelf life | Notes |
|---|---|---|
| Coprogram (general analysis) | 8–12 hours | When stored for >6 hours, errors in determining fatty acids and muscle fibers are possible. |
| Analysis for worm eggs (helminths) | up to 72 hours | For pinworms and roundworms - no more than 48 hours. It is better to donate fresh feces (up to 2 hours). |
| PCR for infections (Clostridium difficile, rotavirus, etc.) | 24 hours | DNA of pathogens degrades during long-term storage. Some laboratories require delivery within 6 hours. |
| For occult blood (hemoglobin test) | 6 hours | Red blood cells are destroyed, the sensitivity of the test drops by 30-40%. |
| Dysbacteriosis (microbiological sowing) | 4–6 hours | Anaerobic bacteria (for example, Bifidobacterium) die without special preservatives. |
⚠️ Attention: If the laboratory uses preservatives (for example, glycerin solution for parasites or Stuart’s transport medium for PCR), storage periods can increase to 5–7 days. Please check this point when receiving a referral!
How to properly store feces in the refrigerator: step-by-step instructions
It is not enough to just put the container on the shelf. To maintain the diagnostic value of the material, follow the algorithm:
- Use a sterile container with a spoon-spatula (issued in the laboratory or sold in the pharmacy). Jars of baby food, mayonnaise, etc. are not suitable. — fats or detergents may remain on their walls.
- Collect stool in a volume of 1-2 teaspoons (about 5–10 g). For loose stools, 3–5 ml is enough. Avoid impurities of urine, water or toilet paper.
- Close the container tightly and sign it (full name, date, time of collection). Use a waterproof marker - the labels on the paper will be wet from condensation.
- Place the container in the middle of the refrigerator (the temperature on the door is unstable due to frequent opening). The optimal area is the shelf above the vegetable drawer (
+4…+6°C). - Do not freeze! The destruction of cellular structures during thawing will make the analysis uninformative.
☑️ Preparation stool for storage
⚠️ Attention: If you donate stool for carbohydrates (for example, to diagnose lactase deficiency in infants), it cannot be stored even in the refrigerator The material must be delivered to the laboratory within 1–2 hours, otherwise the results will be false negative.
What should not be done when storing feces?
Errors at the preparation stage and storage are the main reason for false results. Here are critical prohibitionswhich are ignored by 70% of patients:
- ❌ Store in the freezer - freezing/thawing cycles destroy the shells of parasites and the DNA of pathogens. Exception: special ones (used rarely, only as directed). laboratory). cryopreservatives (used rarely, only as directed by the laboratory).
- ❌ Use unsealed container - oxidation and drying distort the results of the coprogram. For example, the bilirubin level can decrease by 50% in 12 hours.
- ❌ Add water or saline solution - this dilutes the sample and reduces the concentration of markers (for example, calprotectin when analyzing intestinal inflammation).
- ❌ Store next to products with a strong odor (onions, fish, spices) - the pores of the container allow molecules to pass through, which can affect chromatographic tests.
- ❌ Collect feces after an enema or laxatives - this changes the microflora and passage rate, which distorts the results for dysbacteriosis.
What happens if you ignore the rules storage?
When storing feces >24 hours at room temperature:
- False negative result for worm eggs (up to 60% of cases).
- Overestimation of the level of leukocytes in the coprogram (due to cell decay).
- Inability to detect Clostridium difficile (the toxin is destroyed after 8 hours).
- Errors in the diagnosis of pancreatitis (lipase degrades at +20°C).
Features of fecal storage for different categories of patients
Storage periods and conditions may vary depending on age, health status and purposes of analysis. Let's consider the key nuances:
For children (including infants)
Infant feces have high enzymatic activity due to the predominance of a dairy diet. Therefore:
- 👶 For carbohydrate analysis (lactase deficiency) cannot be stored - deliver to the laboratory maximum 1 hour in advance.
- 🧪 For coprograms in children under 3 years of age, the shelf life is reduced to 6 hours (for adults - 12 hours).
- 🦠 If feces are suspected, feces should be examined no later than 4 hours after collection. staphylococcal enterocolitis stool must be examined no later than 4 hours after collection.
For older people
Patients over 65 years of age often experience slow peristalsis and an altered composition of the microflora. This affects the storage rules:
- 🩺 When analysis for occult blood (colorectal screening), the storage period is reduced to 4 hours - due to the increased fragility of red blood cells.
- 💊 If the patient takes anticoagulants (warfarin, aspirin), feces for occult blood must be delivered to the laboratory no later than 2 hours.
For patients with chronic gastrointestinal diseases
With Crohn's disease, ulcerative colitis or celiac disease, the composition of stool changes faster than in healthy people. For example:
- 🔥 For inflammatory bowel diseases (IBD) feces for calprotectin are stored for no more than 3 days at +4°C (instead of the standard 5).
- 🧬 For genetic tests (for example, for mutations NOD2 with Crohn's disease) delivery is required within 24 hours - DNA degrades faster due to high levels of proteases.
Frequent questions about fecal storage: debunking myths
There are many misconceptions around this topic. Let's look at the most common ones:
Myth 1: “Stool can be stored in the freezer if it doesn’t freeze there.”
❌ Reality: Even at -2°C (the typical temperature of the top shelf of a refrigerator with the “fresh zone” function) irreversible changes occur:
- Ice crystals destroy the membranes of helminth eggs.
- The activity of enzymes (for example, enterokinase) decreases by 40%, which distorts the coprogram.
- The DNA of pathogens is fragmented, and PCR can give false negative result.
Myth 2: “If you add a preservative yourself, the stool will last longer.”
❌ Reality: Preservatives (for example, formalin or ethanol) must be used strictly according to laboratory protocol. Incorrect concentration:
- May “blur” the results for dysbacteriosis (will destroy part of the microflora).
- Will cause false positive test for occult blood (due to hemolysis).
- Inactivates some viruses (for example, norovirus), which will lead to missed infection.
Myth 3: “Feces collected in the evening can be donated in the morning - no big deal”
⚠️ Reality depends on analysis:
- For coprograms — permissible if stored at +4°C for no more than 8 hours.
- For PCR for rotavirus — no: the sensitivity of the test drops by 25% after 6 hours.
- For analysis for Giardia - absolutely not: cysts die after 12 hours even in the refrigerator.
⚠️ Attention: Some laboratories (for example Invitro, Helix) indicate on the website individual storage requirements for specific tests. Always check this information in the "Preparation for analysis" section!
FAQ: Answers to popular questions
Is it possible to store feces in the refrigerator in a regular jar if it is washed well?
No. Even after washing with soda or boiling, microparticles of fat, detergents or previous contents remain on the walls of the jar. This will distort the results:
- A false “neutral soap” may appear in the coprogram.
- When analyzing for dysbacteriosis, foreign bacteria (for example, from yogurt residues) will give a false positive result.
Use only pharmacy containers s sealed lid and spoon.
What to do if stool is collected at night, and the laboratory is open only from 8 am?
Optimal algorithm:
1. Place the container in the refrigerator (+4…+6°C) immediately after collection.
2. If the analysis is worm eggs or coprogram, deliver no later than 10–12 a.m.
3. To PCR or occult blood find a laboratory with early appointments (some open from 7:00) or use express courier (for example, in Hemotest or Citylab).
4. If >12 hours have passed, it is better to repeat the collection in the morning.
How to store stool for carbohydrate analysis in an infant?
This analysis cannot be stored even in the refrigerator! Carbohydrates (lactose, glucose) begin to be fermented by bacteria immediately after defecation. For the results to be reliable:
- Collect feces in clean diaper (not a diaper!).
- Transfer to a container for 10 minutes.
- Deliver to the laboratory maximum in 1-2 hours (in some clinics require delivery in 30 minutes).
⚠️ If you don’t have time, use special transport tubes with a preservative (for example, 5% sulfosalicylic acid), but they must be issued by a doctor!
Is it possible to freeze stool for analysis of dysbiosis?
Absolutely not. Freezing kills up to 90% of anaerobic bacteria. (for example, Bifidobacterium and Lactobacillus), which are precisely of interest in this analysis. In addition:
- Ice crystals destroy the cell walls of microbes, and they will not grow on the nutrient medium.
- When thawing, autolysis occurs. (self-destruction) of cells, which distorts the ratio of microflora.
If you do not have time to deliver stool to the laboratory, it is better to reschedule the analysis for another day.
What are the differences between the requirements for storing stool for PCR and for culture for dysbacteriosis?
These are fundamentally different methods, and the storage conditions for them are opposite:
| Parameter | PCR analysis | Cultivation for dysbacteriosis |
|---|---|---|
| Purpose | Detection of DNA of pathogens | Cultivation of live bacteria |
| Max. shelf life | 24 hours (at +4°C) | 4–6 hours (at +4°C) |
| Preservatives | Acceptable (for example, DNA/RNA Shield) | Unacceptable (kill microflora) |
| Sensitivity to freezing | DNA is preserved, but can fragment | Bacteria die |
Conclusion: For PCR it is important save DNA, and for sowing - save living microorganisms. Therefore, never confuse containers!