Background Subtraction: A New Approach to the Assessment of Disease Activity in Crohn's Disease Using99mTc-HMPAO-Labelled Leucocytes

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Abstract
A computer-based technique for the quantification of abnormal bowel uptake in Crohn's disease has been developed and compared with pre-existing clinical, laboratory and scintigraphic methods of assessment. The standard technique for labelling leucocytes with 99mTc-HMPAO is applied. Images were obtained at 40, 120 and 240 min after the injection of radiolabeled leucocytes. The count in the bowel area after subtracting background activity corrected to the injected dose and image acquisition times is the ‘scan score’, an objective measure of disease activity. The scan score is significantly higher in patients with clinically active disease (mean 82.1 ± SEM 13.6) than in those with quiescent disease (24.7 ± 7.0) (p ⩽ 0.005). Optimum separation between active and quiescent disease is achieved with a threshold scan score of 20. The scan score was comparable in small bowel disease (73.3 ± 16.2), large bowel (94.4 ± 33) and disease at both locations (94.1 ± 19.2). The scan score correlated favourably with Crohn's Disease Activity Index (rs = 52, p ⩽ 0.0001), Harvey & Bradshow Simple Index (rs = 0.4, p ⩽ 0.001), serum C-reactive protein (rs = 0.72, p ⩽ 0.001), serum alpha acid glycoprotein (rs 0.67, p ⩽ 0.001), haemoglobin (rs = 0.66, p ⩽ 0.001), platelet count (rs = 0.47, p ⩽ 0.006), albumin (rs = 0.61, p ⩽ 0.0001) and faecal 111Indium excretion (rs = 0.78, p ⩽ 0.001), but not with the ESR (rs = 0.22, p ⩽ 0.4). In 18 patients the scan score was measured serially before and after treatment with elemental diet. A reduction in the scan score by 70% or more is obtained in 82% of patients who achieved clinical remission, but in only one of eight patients who failed to respond to treatment. The scan score provides an objective measure of disease activity in Crohn's disease and correlates with existing clinical and laboratory activity indices. Excellent correlation was also found with faecal 111Indium excretion. The scan score can be used to monitor treatment and may help define remission.