Effect of delay in sampling on haemoglobin determined by faecal immunochemical tests.

BACKGROUND: Faeces must be sampled directly onto guaiac-based faecal occult blood test (FOBT) cards since analysis of specimens collected in traditional faeces containers is inappropriate because degradation of haemoglobin continues after faeces have been passed. Newer faecal immunochemical tests (FIT) are replacing FOBT, but it is likely that the practice of obtaining specimens in traditional faeces collection containers for later analysis will continue. The aim of this study was to assess the effect of delay in stool sampling on FIT.
METHODS: Five specimens of faeces from healthy volunteers, all qualitatively FIT negative, were supplemented with whole blood haemolysate to three different FIT positive concentrations. Each sample was analysed daily after 1-14 days delay using a quantitative latex immunoturbidimetric-based FIT and also after five and ten days delay using a qualitative FIT.
RESULTS: Haemoglobin concentrations fell each day, the rate being generally proportional to the original haemoglobin concentration. After eight days delay, no sample had a haemoglobin concentration >100 ng/mL and, after nine days, no sample had a haemoglobin concentration >50 ng/mL. After five days delay, five of the 15 supplemented faeces with initially positive qualitative FIT had negative FIT; after 10 days, none had positive FIT.
CONCLUSION: False-negative results will occur if sampling of fresh faeces into or onto FIT collection devices is delayed. Laboratories that undertake FIT analyses on faeces collected into traditional containers are likely to miss significant neoplasia. FIT collection devices must be used for sampling fresh faeces.