Understanding Limit of Detection: What 10 and 100 CFU/mL Mean in Practice

What Is Limit of Detection (LoD)?

The limit of detection is the lowest concentration of a target analyte that can be reliably detected by a diagnostic test. In molecular tuberculosis testing, LoD is typically expressed as colony-forming units per millilitre (CFU/mL).

iFIND TBR LoD Parameters

According to manufacturer documentation:

• MTB identification: 10 CFU/mL
• Rifampicin resistance detection: 100 CFU/mL

These values are manufacturer-provided and have not been independently verified.

What Does 10 CFU/mL Mean?

10 CFU/mL means the test can detect M. tuberculosis DNA when there are as few as 10 viable organisms per millilitre of sample. This is a relatively sensitive threshold.

For context:

• Smear microscopy typically detects at 10,000+ CFU/mL
• Culture detects at 10–100 CFU/mL
• Molecular methods like Xpert MTB/RIF detect at ~131 CFU/mL (manufacturer-stated)

The 10 CFU/mL claim for MTB identification would represent high sensitivity, but this has not been independently confirmed.

What Does 100 CFU/mL Mean for Resistance?

The higher LoD for rifampicin resistance (100 CFU/mL) means that resistance detection requires a higher bacterial load than identification. This has practical implications:

• A sample with 50 CFU/mL might test positive for MTB but insufficient for resistance determination.
• Borderline samples may require repeat testing.
• Clinical correlation is essential — a negative resistance result does not exclude resistance in low-bacterial-load samples.

Why Two Different LoDs?

The difference reflects the technical requirements:

• MTB identification targets repetitive elements (IS6110, IS1081) present in multiple copies per genome, making detection easier.
• Rifampicin resistance detection targets the rpoB gene (single copy), requiring more template DNA for reliable mutation detection.

Limitations of These Claims

• These LoD values are manufacturer-stated.
• The actual LoD may vary by sample type (sputum vs CSF vs pleural fluid).
• LoD does not equal clinical sensitivity — real-world performance depends on sample quality, DNA extraction efficiency, and inhibitors.
• Independent verification of these LoD values has not been performed.

Practical Recommendation

Laboratories should:

• Validate LoD claims using their own reference strains and serial dilutions.
• Test multiple sample types relevant to their clinical setting.
• Document any discrepancies between claimed and observed performance.
• Report results with appropriate caveats about manufacturer-provided performance characteristics.