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

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.

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.

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.

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.