In vivo neutron activation analysis has been used to measure bone mineral content in the central skeleton where osteoporotic fractures occur. To be of diagnostic value, the results must be normalized for body size. From data obtained from 74 healthy children and adults up to 55 years of age, we have found that the calcium in the central skeleton is approximately proportional to the cube of the subject's height. The correlation for the adults alone has an r value of 0.81. When data from both adults and children are used, r=0.95. The validity of this cubic height relationship to the Ca concentration measurements has been further substantiated by studies on rats. The total femur calcium content of 110 rats from weanling to 25 weeks of age was proportional to the overall femur (length)3.6 (or (length)2.6 per unit length) with r=0.99.
When the level of Ca content is related to data on normal subjects of the same body size (giving the Calcium Bone Index or CaBI) a good separation is obtained between normal volunteers and osteoporotic subjects. Volunteers who were 20 to 55 years of age had CABI 1.0 ± .12 (SD) while osteoporotics had CaBI 0.69 ± .10 (SD). When the calcium content as determined by in vivo activation analysis is expressed as a CaBI, it provides a powerful tool for the diagnosis of osteopenia. We suggest that all bone measurements, including peripheral ones, be normalized for body size in order to increase their diagnostic value.