Faecal immunochemical tests for haemoglobin (FIT) are now used in asymptomatic bowel screening programmes and also in assessment of patients presenting with lower bowel symptoms.  FIT specimen collection devices have a stick attached to the cap of the tube: this stick has dimples or grooves near the end to collect the correct amount of faeces. Our instructions are simple, namely, “dip the end of the stick into your poo” and “scrape the end of the stick along the sample”, and have pictures of exactly what sample is required. However, many seem surprised at how little faeces is collected, only 2 mg in the FIT used in Scotland for both clinical purposes.  Interestingly, some assume that more must be better and do try very hard to give a little (or a lot) extra in the device!  To date, very little attention has been paid to this aspect of FIT.  Recently, however, a very relevant paper has been published.1

The study showed that infringement of the instructions as to how to collect the sample did have some effects.  Not adhering to instructions regarding inserting the stick properly into the sample, or not visually checking the filling of the dimples or grooves, reduced measured haemoglobin concentrations. Thankfully, the effects on screening diagnostic performance were generally small.

Collecting more sample into the devices than recommended increased concentrations of haemoglobin, in spite of the fact that they have a septum or collar, which clearly does minimise the amount of additional faeces transferred into the buffer in the device.2 Unsurprisingly, this led to detection of more participants with CRC: a good outcome, although additional colonoscopy would be required.

Therefore, this recent evidence suggests that the concern often expressed about participants and patients not following the instructions regarding sample collection for FIT may be somewhat unfounded and that all samples submitted in the correct time frame, including those with that little extra, are satisfactory for FIT.

References

1 Gies A, Gruner LF, Schrotz-King P, Brenner H.  Effect of imperfect compliance with instructions for fecal sample collection on diagnostic performance of 9 fecal immunochemical tests.  Clin Gastroenterol Hepatol 2019 Mar 6. pii: S1542-3565(19)30249-6. doi: 10.1016/j.cgh.2019.03.001. [Epub ahead of print].

2 Piggott C, John C, Bruce H, Benton SC. Does the mass of sample loaded affect faecal haemoglobin concentration using the faecal immunochemical test? Ann Clin Biochem 2018;55:702-5.