When should anal swabs be taken? Analysis of the 6 golden rules for clinical specimen collection

Clinical pathways and practice specifications for accurate diagnosis of gastrointestinal infections

Gastrointestinal infection, as a common clinical disorder, covers gastrointestinal dysentery, viral gastroenteritis, foodborne poisoning and other gastrointestinal dysfunction. According to data from the World Health Organization, about 1.7 billion diarrhea cases occur worldwide each year, 70% of which are caused by the transmission of pathogenic microorganisms through the fecal-oral route. Accurate identification of pathogenic bacteria (such as Salmonella, Shigella) and conditional pathogenic bacteria (such as Clostridium difficile) has become the key to treatment decisions, which depends on the coordinated operation of standardized specimen collection and laboratory testing systems.

When is the gastrointestinal specimen collection necessary? Detailed explanation of clinical indications

When the patient has the following symptoms, it is recommended to initiate etiological testing: abdominal pain that lasts for more than 72 hours accompanied by watery stools or pus and bloody stools (>3 defecations per day), moderate fever with body temperature exceeding 38°C, and fecal microscopy found to be positive for white blood cells or occult blood. It is particularly important to pay attention to the fact that elderly patients may experience atypical symptoms due to slow immune response. At this time, anal swab sampling can be used as a supplementary detection method.

The "gold standard" of microbial detection: six principles of specimen collection

1. Principle of timeliness: collect specimens before antibiotic use, and the optimal window period is 48–72 hours after the onset of symptoms. Studies have shown that premature sampling may lead to insufficient pathogen load (<10^4 CFU/g), and the detection rate will decrease by 40% if the delay exceeds 5 days.

2. Container selection: Use a wide-mouthed sterile toilet box (capacity ≥30ml), and its inner wall hydrophobic coating can prevent specimens from adhering and ensure microbial activity. For bedridden patients, anal swab sampling should be performed using a special swab set containing transport media.

3. Target-oriented culture: routine testing should cover Salmonella (detection rate 12.7%), Shigella (8.3%) and Campylobacter (5.9%). When Clostridium perfringens infection is suspected, pre-reducing medium is required for anaerobic culture.

4. Special pathogen treatment: Clostridium difficile detection requires bedside inoculation into CCA culture medium (cycloserine-cefoxitin-fructose agar) and transfer to an anaerobic environment within 30 minutes. The spore formation rate of these bacteria was positively correlated with the exposure time (r=0.82, P<0.01).

5. Multi-system joint test: Fever patients should collect blood cultures simultaneously (positive rate is about 4.2%), especially when mesenteric lymph node enlargement or liver enzyme abnormalities occur, it suggests that intestinal sepsis may exist.

6. Dynamic monitoring strategy: For hospital infection cases (onset 72 hours after admission) and immunodeficiency patients, it is recommended to send them for examination for 3 consecutive days. Data shows that the sensitivity of a single fecal culture is only 68%, and three tests can be increased to 91%.

Key links of quality control: Full management from sampling to reporting

Common technical errors in clinical practice include: specimens exposed to air for more than 20 minutes (affecting the survival of obligate anaerobic bacteria), non-removal of toilet disinfectant residues (inhibiting microbial growth), insufficient inspection volume (<5g), etc.

7. Standards for monitoring and sample collection of infection cases (I) Process for handling suspected outbreaks of infections in hospitals When medical institutions discover suspected in-hospital infections, laboratory diagnosis becomes a key link. According to the requirements of the "Hospital Infection Management Measures", microbial laboratories should initiate emergency testing plans within 2 hours, focusing on screening of multidrug-resistant bacteria and intestinal pathogenic bacteria.

8. Standardized stool sample collection operation specifications (I) Natural defecation collection method (gold standard) After patients use a special sterile potty to complete defecation, clinical personnel should select fecal specimens containing obvious mucus or bloody components. The recommended collection amount is 2–3 grams of solid samples or 1-3 ml of liquid samples. For feces without obvious abnormal appearance, the "five-point sampling method" is required to collect multiple points in different areas of the feces. This method can maximize the detection rate of pathogens and is especially suitable for the detection of intestinal pathogenic bacteria such as Salmonella and Shigella.

(II) Rectal swab collection technology 1. Standardized operation process for patients with dysfunction or infants with sterilized anal swabs can be used for sampling. During operation, the swab should be moistened with 0.9% sodium chloride solution, and gently inserted into the anus at a 30-degree angle 4-5 cm (2-3 cm for children). After contacting the rectal mucosa, the sampling should be completed by rotating for 3–5 weeks. Studies have shown that standardized operations can obtain a test sample size equivalent to 0.1g of feces.

9. Clinical Application Indications The US CDC Guidelines clearly state that anal swabs are mainly suitable for active screening of CRE (Eczema carbapenem-resistant Nitrobacteria cold). Clinical data show that active screening can reduce the infection rate of multidrug-resistant bacteria by 30-50%. It should be noted that swab samples must be preserved using a dedicated Cary-Blair medium, which can maintain pathogen activity for 72 hours.

10. Biological sample transportation quality control (I) Timeliness Management Standards Fresh feces specimens should be delivered to the laboratory within 2 hours after collection. When the room temperature exceeds 25℃, cold chain transportation boxes must be activated. For special pathogens such as Vibrio cholerae, secondary biosafety protection standards must be implemented, triple packaging systems are adopted and equipped with special personnel to carry them.

(II) Storage Technical Specifications 1. Conventional Storage Plan When the delivery delay exceeds 2 hours, it is recommended to add pH 7.0 phosphate glycerol buffer. This solution was verified by WHO to effectively maintain Shigella survival rate for 48 hours.

 For special media applications, rectal swab specimens must be transported in GN germ-enhancing broth or modified Cary-Blair medium. Clinical comparative trials show that the use of special transportation media can increase the detection rate of pathogenic bacteria by 28.6%.