One of the challenges in clinical diagnostics is the logistics of getting a quality sample from the patient to the laboratory for analysis.
When considering the detection of faecal haemoglobin (f-Hb), this is partly dependent on the technology to be employed. In the days of guaiac based faecal testing, samples were sent in traditional blue-capped “stool pots”. This was clearly wrong, since haemoglobin in native faeces is very unstable (Brown and Fraser1). It degrades rapidly at physiological and ambient temperatures. In passed faeces which contain digestive enzymes, bacteria and fungi, it will degrade even faster.
The moiety being examined in gFOBT is the haem component of the haemoglobin molecule. Young et al.2 demonstrated that, with gFOBT, the degradation was more pronounced in the samples that were not dried, as when collected into a traditional faecal pot, versus a thin dried smeared sample (taken directly onto the gFOBT card). The conclusion was that sampling directly onto the card should be made as soon as possible following defecation. In addition, analysis of gFOBT should be delayed for a few days so that potential interference from plant peroxidases, leading to false positive results, can be minimised.3
With the move to a more sensitive technology based on an immunoassay specific for human haemoglobin, it is vital to stabilise the haemoglobin present in the specimen collection device, prior to analysis, to protect it from degradation and maintain integrity.
Brown and Fraser1 performed a similar study to Young et al2. However, they used both qualitative and quantitative FIT methods to analyse five haemoglobin spiked faecal samples, with daily sampling for up to 14 days. The conclusion was that false negative results for faecal haemoglobin could occur if sampling fresh into the tubes or onto the cards of FIT collection devices is delayed.
With the release NICE Guidance DG30, now focussing on the benefits of the application of FIT as a means to triage patients with lower gastrointestinal (GI) symptoms, it is important that any loss of f-Hb is protected.
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- CRUK Resources