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Usually, more than one specimen is collected from a patient. Therefore, it is not necessary to salvage a contaminated specimen if other specimens from the same patient are positive and not contaminated. However, if it is critical to have the results of a particular specimen that was contaminated, the broth may be reprocessed.
A high contamination rate indicates improper decontamination procedure. At the same time, too low contamination indicates over-treatment of the specimen that could also lower the culture positivity rate or increase the detection time. If in the MGIT it is more than 7-8%, then the decontamination procedure is not satisfactory and corrective measures should be taken:
Detection time can range from 24 hours to six weeks for MGIT and 8 weeks for LJ medium. If the average detection time is significantly longer, these instructions need to be followed:
Digestion/decontamination procedure:
If there is an overall decrease in the culture positivity rate, the following parameters need to be investigated:
Laboratories need a mechanism to determine possible causes of false results (e.g., personnel logs, lot numbers used). Practices should be evaluated to determine where changes can be made. Most episodes of false results are recognized only after a review of laboratory records, including genotyping results. Laboratories need a review process to detect false cultures earlier.
False negatives are test results reported as negative for a Mycobacterium species present in the patient specimen
Practices that can lead to false negative cultures are :
False positives are test results reported as positive for a Mycobacterium species not present in the patient specimen.
Not all false positives are due to laboratory cross-contamination.
The growth of contaminant bacteria results in positive fluorescence. Therefore, it is important to observe all fluorescent positive MGIT tubes visually for turbidity and to make an AFB smear. If an MGIT tube broth is heavily turbid, contamination is suspected even if the AFB smear is positive.
Liquid Culture Contamination and Sources of Contamination
MGIT results are interpreted as described in Flowcharts 1-3: