Sputum from a suspected pulmonary TB patient is tested on Xpert MTB/RIF Ultra. The report reads 'MTB Detected (Trace), Rifampicin resistance not detected'. In the semiquantitative reporting scheme, what does the 'Trace' category signify?
- A The lowest detectable bacillary burden, close to the assay limit of detection ✓
- B Cross-contamination during sputum collection
- C The specimen failed the sample processing control
- D A probe pattern suggesting borderline rifampicin resistance
Correct answer: A. The lowest detectable bacillary burden, close to the assay limit of detection
Explanation
Ultra reports semiquantitative categories from High through Medium, Low, Very Low and finally Trace, with Trace representing the smallest amount of detectable MTB DNA, just above the limit of detection. It typically reflects paucibacillary disease, such as in HIV co-infection, children or extrapulmonary samples. C failed control would be reported separately as invalid or error, and rifampicin resistance is reported independently of bacillary load.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.