The SCORE index and bone mineral density assessments

Introduction
The SCORE index (Simple Calculated Osteoporosis Risk Estimation) was devised by Lydick and colleagues[1] as a simple technique to predict which individuals are likely to have osteoporosis as defined by measurements of bone mineral density (BMD) using World Health Organization (WHO) criteria. They sought to determine if in practice this new index may be used to predict which individuals referred for BMD assessment would be unlikely to have osteoporosis of the femoral neck. They described the design of this index and its validation in 259 female peri- or post-menopausal female patients derived randomly from 106 US centres.

In our study, we looked at another ethnic group of patients using a different health-care system and in a clinical practice setting to see whether the index has a wider validity. We chose to assess patients already referred for BMD measurements to determine if in clinical practice this index may be used to predict which individuals so referred would be unlikely to have osteoporosis of the femoral neck.

WHO has recommended the use of a T-score to normalize data, at least in white patients. Thus, the T-score relates to the bone density of a given individual to the notional peak bone density noted for a generic North American population of white women. A T-score of 0 represents a bone density equal to the mean peak bone mass, a T-score of –1 represents 1 standard deviation below this, and a T-score of –2.5 has been designated by WHO as representing osteoporosis.[2] Whether treatment should begin at this point, prior to that, or subsequently depends a great deal on the patient and remains controversial.

Methods
We constructed a SCORE index for 989 consecutive patients seen at one of two radiology clinics in Edmonton. Only peri- or post-menopausal female patients were included. They had been referred by a broad range of physicians, including family doctors, internists, rheumatologists, and others. Approximately 95% of the subjects were white, 4% Asian, and 1% black. Calculation of the SCORE index is explained in the Table 1. Lydick and colleagues recommended that a score of less than 6 be used as a cut-off point.

This study received approval of the Research Ethics Board of the Faculty of Medicine at the University of Alberta.

Results
Figure 1 illustrates the combined results for the femoral neck. Only one patient (0.1%) who had a BMD T-score of –2.5 SD would have been missed had the SCORE index been used as a screen to determine the likelihood of an osteoporotic result. Thus 221 individuals (22.3% of the total) who had a T-score of >–2.5, i.e., non-osteoporotic, could have been excluded by pre-screening with the SCORE index. The percentage was even higher in the 50 to 59 age group, where 43% had a normal SCORE index and would not have been tested had this index been used as a screen. No patients with osteoporosis would have been missed, although seven subjects with significant osteopenia, i.e., a T-score between –2 and –2.5, would not have been selected.

All but one (0.1%) of those with a BMD of the femoral neck outside the osteoporotic range had a normal SCORE index. The specificity of the SCORE was relatively low; 77% of those individuals with an abnormal index had a T-score of better than –2.5 (see Figure 1). Nevertheless, its sensitivity was high at over 99.5%. The negative predictive value, i.e., the probability of no osteoporosis with a negative test, was, in our patient population, over 99.5%.

Discussion
The role of BMD measurement in the assessment, diagnosis, and management of osteoporosis remains controversial. Canadian guidelines have suggested restricting its use to specific indications,[3] and an international group recently restated the importance of BMD quantitation[4] where possible and when advisable. While a BMD measurement may be requested for a variety of legitimate reasons, our results suggest that the use of the SCORE index in our patient population would allow us to recognize the low likelihood of osteoporosis in about 25% of those patients referred, and thereby reduce the need for BMD measurement. A false negative rate of less than 1% would seem acceptable.

How to calculate the SCORE index

Dr Morrison is a professor emeritus at UBC and Vice President of the Diagnosticare Imaging Centre at the University of Alberta. Dr Russell is a professor of medicine at the University of Alberta.