CDST_LT: False culture results

False Positive MGIT Cultures

Content

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.
All laboratories are capable of producing false positive results.

Practices that can lead to false positive cultures are:

Inadequate sterilization of instruments such as bronchoscopes
Mislabeling at the time of collection or time of accessioning
Use of contaminated water for specimen collection or laboratory procedures
Shared reagents and dispensers
Opening more than one specimen container at a time while processing
Mix-up of testing samples or lids
Failure to take precautions to minimize aerosol production

Resource

Mycobacterial Culture

Assessment

Question 1	Answer 1	Answer 2	Answer 3	Answer 4	Correct Answer	Correct Explanation	Page id	Part of Pre-Test	Part of Post-Test
False positives are test results reported as positive for a Mycobacterium species not present in the patient specimen.	True	False			True	False positives are test results reported as positive for a Mycobacterium species not present in the patient specimen.		Yes	Yes
Question 2
The use of contaminated water for specimen collection or laboratory procedures is one of the reasons for false positives.	True	False			True	Among several reasons, the use of contaminated water for specimen collection or laboratory procedures is one of the reasons for false positives.		Yes	Yes

False Negative MGIT Cultures

Content

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 :

Extended exposure to NaOH during processing
High pH (insufficient neutralization during processing)
Growth may be inhibited by:
- addition of too much PANTA
- loss of CO₂ from MGIT vial headspace

Resource

MGIT 960 Growth Detection Oct09.ppt (1.46 MB)

Assessment

Question 1	Answer 1	Answer 2	Answer 3	Answer 4	Correct Answer	Correct Explanation	Page id	Part of Pre-Test	Part of Post-Test
False negatives are test results reported as negative for a Mycobacterium species present in the patient specimen.	True	False			True	False negatives are test results reported as negative for a Mycobacterium species present in the patient specimen.		Yes	Yes
Question 2
Extended exposure to NaOH during processing results in false negatives.	True	False			True	Extended exposure to NaOH during processing results in false negatives.		Yes	Yes

Impact of False MGIT Culture Results

Content

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.

Consequences of false positives which impact on clinical care of patients:

1. Patients may be managed incorrectly in the following ways.

Unnecessary treatment and toxicity
Unnecessary isolation, hospitalization, and healthcare costs
Emotional repercussions to the patient
Unnecessary contact investigations

2. Credibility of the laboratory, hospital, or clinician may be questioned.

3. It May increase laboratory workload and testing costs.

Resource

Mycobacterial Culture.

Assessment

Question 1	Answer 1	Answer 2	Answer 3	Answer 4	Correct Answer	Correct Explanation	Page id	Part of Pre-Test	Part of Post-Test
False positive results may lead to unnecessary treatment and toxicity.	True	False			True	False positive results may cause unnecessary treatment of the patient and cause toxicity.		Yes	Yes
Question 2
Due to false positive results credibility of the laboratory, hospital, or clinician may be questioned.	True	False			True	False positive results affect the credibility of the laboratory, hospital, or clinician.		Yes	Yes