A critical assessment of bone scan quantitation (bone to soft tissue ratios) in the diagnosis of metabolic bone disease.

Accurate quantitation from the bone scan image of skeletal uptake of radiopharmaceutical would be of value in the assessment of patients with metabolic bone disease. Repeat measurements of bone to soft tissue (B/ST) ratios on the one set of images were made for 103 subjects, a) by the same observer using lumbar vertebra 2 for the area of bone; b) by the same observer using lumbar vertebra 2 then lumbar vertebra 4; c) by two observers both using lumbar vertebra 2. The median difference between repeat measurements by the same observer was well under 1% but the 5-95 percentile range was -13 to +14%. Between the two observers there was a median difference of 10.6% with a 5-95 percentile range of -11 to +44%. We also measured B/ST ratios in 150 control subjects and 139 patients with various metabolic bone disorders. While statistically significant differences for B/ST ratios were found between the osteomalacia, renal osteodystrophy, Paget's groups, and the control population (P less than 0.001 in all cases), there was appreciable overlap between individual patient results and the control range. It is concluded, therefore, that measurement of B/ST ratios for the individual is of limited value in clinical practice.