Patient Sample Collection

OBJECTIVES FOR SAMPLE COLLECTION SECTION

For each of the following specimens, you should be able to indicate the appropriate collection method and transport conditions that are recommended for optimal recovery of microorganisms:

1. Throat culture (swab)

2. Culture of an anaerobic abscess

3. Throat washing for viruses

4. Transport of urine specimen to a reference laboratory

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Normal flora can be problematic in the collection of medical samples and can confuse diagnoses. Human body surfaces and cavities are ideal environments for some classes of microorganisms. Many of the organisms used in the laboratory portion of this course will be obtained by swabbing skin surfaces, body folds, throat surfaces or nasal passages. This approach emphasizes the fact that "normal" is not identical for all people, and that each individual human being is an environmental niche for his or her own unique set of commensal microorganisms. Students should consider human disease that can be caused by the normal flora (bacteria; sometimes fungi), why some individuals have more problems with foreign microorganisms than others, and how disease-causing organisms move between human populations.

Throat Swab Sample

1. Using one of the dry sterile swabs, perform a throat culture on a cooperative fellow student (see below).

Ask the culturee to open the mouth widely and say a long "ah". The tongue should be gently depressed with a sterile tongue blade. The swab is then gently passed over the tongue and into the posterior pharynx. The mucosa behind the uvula and between the tonsils should then be gently swabbed with a back-and-forth motion. Two samples can be collected simultaneously using the method shown in the diagram above. This allows one to be used for direct antigenic testing for organisms like Streptococcus pyogenes, and the other to be used in plating the sample for culture (next step). The technique is especially useful for young children, who might resist a second sample collection after the unpleasant sensation caused by the first swab.

2. Streak the swab sample carefully onto a Blood Agar plate (BAP), pulling a three-part pattern of inoculation to obtain isolated colonies and incubate for 24-48 hours in the presence of CO₂.

Tooth Swab Sample

1. Using one of the dry swabs, collect a sample of your own oral flora. Swab the tooth surface in a manner similar to the technique you use in brushing your teeth. Be sure to swab at the gum line and in the folds surrounding the molars at the back of the mouth.

2. Streak carefully onto a Blood Agar plate (BAP) as described in #2 above and incubate for 24-48 hours in the presence of CO₂. Observe plates for evidence of hemolysis. Be sure to observe other students’ plates to see a wide range of results.

Sputum Sample

A simple, easy-to-collect sample, sputum has the disadvantage of contamination with normal oral flora. This method is only effective for upper respiratory infections and is insensitive for the recovery of lower respiratory pathogens.

Sputum or Urine Specimens for Tuberculosis

First early morning samples are recommended, not to exceed three successive 24 hour periods. Bacteria are present then in a more concentrated form, and successful acid-fast staining can easily be performed when Mycobacterium tuberculosis is suspected.

Blood Culture

At least two sets should be collected per septicemic episode, spaced 45 minutes or more apart. In the case of typhoid fever, blood cultures during the first week of illness produce optimal recovery of organisms. Extreme care must be given to disinfection of the venipuncture site to minimize contamination with normal flora.

Antibiotic Drug Levels

Analysis is done to determine peak and valley concentrations of drug. The goal is to maintain a plasma drug level 2-3 times the minimum inhibitory concentration (MIC) of the drug at the valley period.

Stools for Bacterial Infection

Rapid transport of specimen to the lab is essential because many enteric pathogens are sensitive to prolonged contact with acidic stools. Antibiotic-containing and pathogen-selective media are used to limit the growth of normal intestinal flora (such as SS Agar for Salmonella and Shigella). Many of the enteric pathogens are causative of reportable conditions for which records are maintained (notifiable diseases). Reports are made directly to the State Health Department by the attending physician. Health Departments in turn, report to the Centers for Disease Control and Prevention in Atlanta, GA.

Stools for Ova and Parasites

One analysis per 24 hours is sufficient, maximum of 3 analyses. Yield on specimens collected after the third day of hospitalization is usually negligible. Direct examination of concentrated samples is performed to identify parasites.