White cell scanning for inflammatory bowel disease: are biochemical markers useful referral criteria?

The aim of this study was to determine whether biochemical markers for inflammation could prove effective in identifying the most appropriate patients with suspected inflammatory bowel disease (IBD) for labelled white cell scanning. One hundred and twenty-five patients referred for 99mTc-HMPAO labelled white cell scans were investigated. The values of C-reactive protein (CRP), antichymotrypsin (ACT) and acid glycoprotein (AGP) were measured in 73 patients, AGP and CRP in 10 and CRP only in a further 42. Sensitivity and specificity of each test were calculated using the white cell scan result as the 'gold standard'. ACT had the highest specificity (1.0), but the lowest sensitivity (0.27) of the three markers. CRP (using specified limits) had the lowest specificity (0.67) and the highest sensitivity (0.79). The corresponding values for AGP are 0.87 and 0.48. The low sensitivity of ACT and AGP preclude them from being useful referral criteria. CRP (using specified limits) is the most sensitive marker, but not sensitive enough to be useful as a referral indicator. However, by lowering the upper limit of normal to 5 mg.l-1, the sensitivity of the test increases to 0.96. Using this threshold to select the patients, 30% would not have been scanned and only one patient out of the 22 with IBD would have been missed. Where there is high demand for white cell scans this may provide a useful strategy for rationalizing the requests with minimal consequence on clinical management.