False-Positive Investigation Toolkit: Laboratory Areas for Investigations

The laboratory specimen processing log is a record of information about when specimens are initially processed and tested. Review the specimen processing log for accuracy and notes of incidents that may have occurred during processing. Pay attention to possible clerical errors related to the order of the specimens processed or transcription errors. Also, determine if a large number of specimens were processed in one batch. If another department is responsible for accessioning patient specimens, it may be beneficial to inquire about the process used to organize, label, and enter information or data about patient specimens1.

Contamination from a positive to a negative specimen is more likely to occur in specimen processing runs in which MTBC positive specimens are processed alongside new patient specimens. Additionally, highly AFB smear-positive specimens can also be the contamination source for a false-positive result due to the high bacterial burden in the smear-positive specimen23 .

Genotyping analyzes the genetic material (e.g., DNA) of MTBC. The genotype of a suspected false-positive isolate may match the genotype of the isolate that was the suspected source of the contamination3 . When possible, another specimen from the patient(s) should be sent for genotyping.

Matching genotype results of a new MTBC positive patient with a previous MTBC positive patient or multiple MTBC positive patients with matching genotypes may suggest that there was cross-contamination during specimen processing23 .

When multiple patient specimens in a diagnostic series are submitted over time with varying test results, this may suggest that a possible error in testing has occurred. If the patient isolates have different genotypes, then this is likely the result of contamination. However, this is not always the case.

If an isolate from a patient specimen has a genotype that matches a laboratory PT or QC strain, this may identify the source of contamination that caused the false-positive result. PT samples and positive QC strains to be used as controls should not be processed with patient specimens. Procedures should be reviewed if the same BSC or pipette was used to process PT specimens during a previous batch or if the same bottles of reagents were used for both PT or QC and patient specimen processing.

Laboratory and clinical results should be consistent. Clinical symptoms of TB disease are a good indicator of a true positive specimen result, while the absence of clinical symptoms would suggest a false-positive result in most cases24 . On occasion, false-positive results can occur in a patient with TB disease.

It is important to ensure that a patient collection container is associated with the correct patient specimen. Patient specimens collected on the same day or in the same location increases the potential for specimen mix-up. This is especially concerning if patient specimens previously identified as MTBC are collected with other patient specimens on the same day or at the same time at the same location. Contamination has been noted from the same location using the same collection instruments such as bronchoscopes56.

It is important to properly label patient collection containers. Clerical errors can occur when specimens are being prepared for submission to the laboratory. Patient information and specimen collection containers should be checked for accuracy and to ensure the patient information matches.

Laboratory-specific data should include a record of culture growth rates for MTBC-positive specimens. False-positive specimens, resulting from carry-over during specimen processing, will most likely have fewer organisms compared to a known MTBC-positive specimen. Slower growth rate and lower colony counts have been observed with false-positive specimens23 .

All laboratory standard operating procedures should be followed. Excessive work schedules, reduced staffing, and large specimen processing batches, can all contribute to errors and potential contamination of specimens. Regular observation of technique and review of best practices should be implemented for all staff7. It may be beneficial during an investigation to speak with staff who performed the testing to understand if anything unusual with the specimen processing batch occurred.

Nonconforming event reporting keeps track of irregular events that occur in the laboratory (e.g., departure from a documented requirement or procedure such as a regulation, statute, policy, procedure, customer requirement). These events could contribute to either a false-positive result or ongoing laboratory issues that need to be resolved.

Reagents, media, and kits used for processing patient specimens need to be acceptable for use and not expired. Expired reagents, media, or kits can affect data quality and may lead to improper results

In-house review of documented reagent/media/kit lot issues

Recurring errors with testing could suggest a problem with a reagent, media, or kit component. Ensure that reagents, media, and kits are stored at the proper temperature and that any in-house preparation is performed per manufacturer’s instruction. When reagent, media, or kit materials expire, they may no longer work effectively, which can lead to inaccuracies that affect data quality.

Contact reagent/media/kit manufacturers to determine if there are reported lot issues

There may be unknown manufacturing errors in certain reagent, media, or kit components. Manufacturers should be contacted to determine if there have been issues with particular lots of reagents, media, or kits and other user complaints.