6.6 Discordant results

Interpretation of discordant results

When more than one test is performed on an individual, the results may be discordant, which can lead to diagnostic dilemmas. It is important as a first step to exclude possible administrative errors (specimen mislabelling or switching) by reviewing the tests conducted at or around the same time. Reviewing the patient history is also useful because that may highlight a long history with a possibility that an individual may be harbouring more than one strain of Mtb. Technical issues could also be the cause – although the tests are expected to perform similarly, they are not identical; in addition, sample-to-sample variability complicated the issue. Each discordant result will need to be investigated on a case-by-case basis. A few examples are outlined below.

  1. Where the mWRD (e.g. Xpert Ultra) result is “MTBC detected”, and the follow-on FL-LPA result is “MTB not detected” or “uninterpretable”:
    1. The mWRDs recommended for detection of TB have a lower LoD than the FL-LPA; thus, FL-LPA may fail to detect TB in mWRD-positive samples that contain few bacilli. For example, it is estimated that only about 80% of specimens with “MTBC detected” by Xpert MTB/RIF will generate an FL-LPA result that can be interpreted.
    2. The initial mWRD result should be used to guide treatment decisions, pending additional testing.
    3. Follow-up actions may include submitting a specimen for culture and a molecular or phenotypic testing of the recovered isolate, and evaluating the possibility of laboratory or clerical error.
  2. Where the initial mWRD result is “MTBC detected, RIF resistance not detected” and the sample is RIF resistant by FL-LPA:
    1. Treatment decisions should be based on the FL-LPA result (i.e. based on the worst-case scenario).
    2. This result is expected to be rare because both assays interrogate the same region of the rpoB gene. There have been reports of mWRD RIF-susceptible and FL-LPA RIF-resistant discordances, but the data are too sparse to assess how often this occurs.
    3. FL-LPA is more sensitive for identifying RIF resistance than most mWRDs in heteroresistant populations (i.e. mixtures of susceptible and resistant bacteria). The test includes hybridization probes specific to both the common mutated and the wild-type sequences in the bacterial genome. If the Xpert Ultra is used, a review of the probe melting temperature curves may help to identify heteroresistant populations (e.g. dual peak).
    4. Follow-up actions may include DNA sequencing, conducting phenotypic DST, and evaluating the possibility of laboratory or clerical error.
  3. Where the MC-aNAAT result is “MTBC detected, RIF resistance not detected, INH resistance detected” but the result is “INH susceptible” by LC-aNAAT:
    1. This result is expected to be rare because both assays interrogate the same region of the katG and inhA genes.
    2. A more likely reason is the existence of heteroresistant populations (i.e. mixtures of susceptible and resistant bacteria), especially in high-burden settings where the force of infection is high. A review of the LC-aNAAT probe melting temperatures (92) may identify such a possibility (e.g. dual peak).
    3. Follow-up actions may include DNA sequencing, conducting phenotypic DST, and evaluating the possibility of laboratory or clerical error.
    4. The risk of Hr-TB should be reassessed; if the reason is found not to be a high risk of Hr-TB or administration errors (e.g. mislabelling), treatment decisions should cover the worst-case scenario and be based on the MC-aNAAT result.
  4. Where the MC-aNAAT result is “MTBC detected, RIF resistance not detected, INH resistance not detected” and the sample is INH resistant by LC-aNAAT:
    1. Treatment decisions should be based on the LC-aNAAT result (i.e. treat based on the worst-case scenario).
    2. This result is expected to be rare because both assays interrogate the same region of the katG and inhA genes. However, the LC-aNAAT is more sensitive for INH detection because it includes additional gene targets (fabG1 and oxyR-ahpC intergenic regions).
    3. The existence of heteroresistant populations (i.e. mixtures of susceptible and resistant bacteria) is another possible reason, especially in high-transmission settings. A review of the LC-aNAAT probe melting temperatures may identify such a possibility (e.g. dual peak).
    4. Follow-up actions may include DNA sequencing, conducting phenotypic DST, and evaluating the possibility of laboratory or clerical error.
  5. Where targeted NGS detects resistance and other molecular tests show susceptibility (or vice versa):
    1. Results should be checked to see whether the mutation detected by targeted NGS was in a region not covered by the other molecular test. If that is the case, the targeted NGS should be considered to be the final result.
    2. Results should be checked to see whether the mutation is a synonymous mutation by targeted NGS. If that is the case, it would indicate that the other molecular test result is incorrect.
    3. Results should be checked to see whether targeted NGS detected heteroresistance. If that is the case, the most resistant profile should be used for clinical management. Targeted NGS is better at resolving heteroresistance than other molecular tests.

Navigation du livre