Investigation of a problem with external quality assurance samples for faecal occult blood.

Background: It is vital that laboratories participate in External Quality Assurance (EQA) programmes, but results from such schemes do not necessarily help ensure assays are 'in control'. We describe a series of experiments undertaken to explore apparent poor performance in an occult blood EQA scheme that manifested as a series of false-negatives. As a consequence of our laboratory misclassifying some EQA samples, we decided to design a simple sensitivity study that would reaffirm confidence in our testing procedures.

Methods: In order to verify analyst reliability and analytical sensitivity, a series of 10 faecal samples were supplemented with increasing amounts of whole blood (0-1.0 mg/Hb/g faeces) and these were then tested independently by four different operatives. EQA samples which our laboratory misclassified were shared with and tested by a second laboratory and results compared.

Results: The results demonstrated consistency in interpretation among the four 'testers' and also suggested that the sensitivity of the method is 0.7 rather than 0.6 mg/Hb/g faeces as claimed by the manufacturer. The second laboratory obtained identical results as our laboratory for the EQA samples which our laboratory had misclassified.

Conclusions: Occult blood EQA scheme results might not give a true reflection of a laboratory's ability to provide reliable results. We would encourage any laboratory observing relative poor performance to undertake local sensitivity studies and repeat testing of EQA samples if in any doubt.