The FIT is a stool-based test that uses antibodies to detect human haemoglobin (Hb) in faeces. Unlike the older guaiac-based faecal occult blood test (gFOBT), FIT is specific to human blood and does not require dietary restrictions. This specificity reduces false positives from non-human dietary sources and enhances the test’s accuracy for detecting bleeding in the lower gastrointestinal tract.
The effectiveness of the FIT is measured by its sensitivity (ability to correctly identify those with disease) and specificity (ability to correctly identify those without disease). These metrics can vary based on the haemoglobin concentration threshold set for a positive result.
The NBSP in New Zealand has adopted a cutoff of 40 micrograms or 200 nanograms. This decision balances sensitivity and specificity to optimize cancer detection while minimizing unnecessary colonoscopies.
- Rationale:
- Enhanced Detection: Lowering the cutoff increases the test’s sensitivity, allowing for earlier detection of CRC.
- Resource Management: While a lower cutoff may lead to more positive results and subsequent colonoscopies, it aims to catch more cases at an earlier, more treatable stage.