False Positive Investigation Toolkit: Laboratory Monitoring

Patients positive for MTBC should have consistent results in the diagnostic series. Results that are discordant within the set of diagnostic cultures could be indicative of a false-positive result. 12 Having only one positive liquid or solid media MTBC result out of multiple liquid or solid cultures for the same patient could be an indication of a false-positive result. Also, single positive specimens have been a sensitive predictor of cross-contamination in past investigations.345

Studies have demonstrated that most diagnostic cultures positive for MTBC will be detected within an average incubation time of 28 days67 . A culture that has delayed growth on either liquid or solid media and low colony counts on solid media may be a cause for concern89. Time to positivity can be delayed if bacterial load in the specimen is low. A cluster of positive cultures that appear late with scanty growth (<10 colonies on solid media) or a delay in recovering mycobacteria in a liquid system may indicate that contamination occurred during processing8.

Having a negative AFB smear control slide test positive indicates contamination of the control, reagents, or the slide. Further investigation is needed before reading patient AFB smears. Similarly, AFB growth on culture media for the negative control indicates that contamination occurred in the processing batch or that the media itself was contaminated. All culture results from that processing batch should be reviewed since the results are invalidated by the positive result in the negative control. If results have been reported, corrected reports should be issued and providers should be notified.

Routine laboratory monitoring of the incidence of each species identified (MTBC and NTM) could detect an increase in a rare species not commonly seen in the laboratory, which could be suggestive of an increase in laboratory contamination. Similarly, a sudden increase in the percentage of a species typically found to be environmental contaminants (e.g., M. gordonae) could be a sign of contaminated water or reagents10.

If a laboratory experiences an unusual increase in MTBC isolates with identical drug resistance patterns, this may suggest possible false-positive results. The laboratory should determine whether cross-contamination may have occurred during specimen processing or culture manipulation set up for isolate identification or drug susceptibility testing1112 . Laboratories should collaborate with the program’s surveillance or epidemiology team to assess whether there is a suspected cluster with epidemiologic links, or if the same patient was tested but with different identifying information (e.g., mislabeled). Genotyping results should also be reviewed for each of the isolates.

Unusual genotyping results, such as results from patient specimens that are identical to laboratory PT or QC strains, indicate false-positive results. Particular strains of MTBC have been chosen for PT and QC strains for differentiation from strains that circulate in the general population. When these strains are found during genotyping, it is more likely that these results are an indication of a false-positive MTBC result. The exception is whether the isolate could be from a possible laboratory exposure.

An increase in the percentage of positive MTBC culture results in a laboratory could be a sign of false-positive results, especially if there is no other feasible explanation (e.g., an outbreak). Any increase in culture positivity should be investigated. Routine monitoring of laboratory indicators, such as total number and percent of positive cultures belonging to MTBC, will allow for proactive assessment. Additionally, monitoring these data and maintaining communication with the TB Program could assist in explaining test results. Genotyping results should also be reviewed when an increase in positive culture results are investigated.

A single positive NAAT result would be considered suspicious if AFB smear and culture were negative13. Results need to be correlated with other diagnostic findings and patient history. Single positive NAAT results could be due to contamination or the detection of non-viable organisms in patients on treatment.

Reviewing a patient’s previous test results when setting up current testing will help identify whether the patient has been previously positive for MTBC. By identifying this early on, the current specimen can be placed appropriately within the processing batch to minimize potential cross-contamination.

Reviewing a patient’s previous test results when reporting out a positive result report will help to determine whether the patient is a new positive or is consistent with previous results for that patient.

When a specimen is identified as positive for MTBC, it is good practice to determine if other positive patient specimens were in the same batch (e.g., processing, identification or drug susceptibility testing). This helps to rule out or to identify the possibility of cross-contamination from other known MTBC positive specimens processed in the same batch. Studies show that multiple positive patient cultures from the same processing batch may be associated with an increased likelihood of false-positive results.1 However, it is important to determine a laboratory’s normal positivity rate as a baseline for comparison.

Many laboratories perform monthly or quarterly review of all MTBC results to monitor for inconsistent or remarkable results. Unusual patient or laboratory findings could include but are not limited to: same collection facilities, clusters of positive growth for multiple patient specimens processed on the same day, no clinical symptoms associated with positive culture results, or genotypes matching laboratory controls.

Maintaining accurate and complete records including specimen processing logs, lot numbers, or quantity of specimen received can assist in identifying potential causes of an unusual or nonconforming event that leads to a potential false-positive. Logs or notes could provide critical details, which are especially important when the investigation is conducted weeks later, after an MTBC identification or unusual drug-resistance pattern has been determined. Using this information could prevent erroneous reporting or assist a false-positive investigation.

While it is advisable to monitor laboratory procedures and policies proactively, laboratories may receive inquiries from a healthcare provider or TB Program regarding test results. It may be necessary to initiate an investigation based on these inquiries to provide as much information to the healthcare provider or TB Program for appropriate patient care. Development of a laboratory procedure to review specimen processing logs, patient history, susceptibility patterns, and genotyping for suspected false-positive cases should be implemented.