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

This version was published on 1 November 2008

Ann Clin Biochem 2008;45:604-605
doi:10.1258/acb.2008.008024
© 2008 Association for Clinical Biochemistry

 

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Louise F Brown1 and
Callum G Fraser2


1 Department of Biochemical Medicine, Ninewells Hospital and Medical School;
2 Scottish Bowel Screening Centre Laboratory, Kings Cross, Dundee, UK


Corresponding author: Louise F Brown. Email: l.brown3{at}nhs.net


Background: Faeces must be sampled directly onto guaiac-based faecal occultblood test (FOBT) cards since analysis of specimens collectedin traditional faeces containers is inappropriate because degradationof haemoglobin continues after faeces have been passed. Newerfaecal immunochemical tests (FIT) are replacing FOBT, but itis likely that the practice of obtaining specimens in traditionalfaeces collection containers for later analysis will continue.The aim of this study was to assess the effect of delay in stoolsampling on FIT.

Methods: Five specimens of faeces from healthy volunteers, all qualitativelyFIT negative, were supplemented with whole blood haemolysateto three different FIT positive concentrations. Each samplewas analysed daily after 1–14 days delay using a quantitativelatex immunoturbidimetric-based FIT and also after five andten days delay using a qualitative FIT.

Results: Haemoglobin concentrations fell each day, the rate being generallyproportional to the original haemoglobin concentration. Aftereight days delay, no sample had a haemoglobin concentration>100 ng/mL and, after nine days, no sample had a haemoglobinconcentration >50 ng/mL. After five days delay, five of the15 supplemented faeces with initially positive qualitative FIThad negative FIT; after 10 days, none had positive FIT.

Conclusion: False-negative results will occur if sampling of fresh faecesinto or onto FIT collection devices is delayed. Laboratoriesthat undertake FIT analyses on faeces collected into traditionalcontainers are likely to miss significant neoplasia. FIT collectiondevices must be used for sampling fresh faeces.


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