British Journal of Radiology (2006) 79, 464-467
© 2006 British Institute of Radiology
doi: 10.1259/bjr/47352663
Heel bone densitometry: device specific thresholds for the assessment of osteoporosis
E McCauley, MSc1,
A Mackie, PhD1,
D Elliott, RGN, ONC3 and
A Chuck, FRCP2
1 Regional Medical Physics Department and, 2 Department of Rheumatology, University Hospital of North Durham, North Road, Durham DH1 5TW, 3 Durham and Chester-le-Street Primary Care Trust, John Snow House, Durham University Science Park, County Durham DH1 3YG, UK
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Abstract
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A diagnosis of osteoporosis is facilitated by bone mineral density (BMD) measurement of the lumbar spine and hip using dual energy X-ray absorptiometry (DXA), interpreted in accordance with criteria published by the World Health Organization (WHO). The use of peripheral DXA is growing in primary care and guidance on its use has recently been published by the National Osteoporosis Society (NOS), recommending a triage approach using thresholds specific to each type of peripheral device. However, no data currently exist for the Norland Apollo heel densitometer (Cooper Surgical, Trumbull, USA). 215 women between 50 years and 75 years of age (mean age 64.6 years) referred for hip and spine BMD measurements also had a heel BMD measurement. Device specific upper and lower thresholds were calculated for the Norland Apollo heel densitometer to give a 90% sensitivity and 90% specificity for osteoporosis at the hip or spine. Patients with a heel T-score of above –1.2 are very likely to have normal bone density on axial densitometry, whilst patients with heel T-score of below –2.2 are very likely to have osteoporosis at the hip or spine. Patients whose measurements lie between the thresholds should be referred for axial DXA.
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Introduction
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A diagnosis of osteoporosis or osteopenia can be made using dual energy X-ray absorptiometry (DXA) of the lumbar spine or femur. The defining criterion for the diagnosis is the value of the T-score, which is the number of standard deviations the measured bone mineral density (BMD) is above or below the young adult reference BMD. According to the World Health Organization (WHO) criteria [1], the patient is osteoporotic if the T-score 
–2.5 and osteopenic if the T-score >–2.5 but –1.
Axial densitometers, which are designed to measure BMD at the hip and/or spine, are large, expensive and require a permanent location, ordinarily in a secondary care setting. Significant research into the development of alternative methodologies has resulted in a plethora of techniques and alternative measurement sites [2]. The measurement of peripheral bone density is growing outside secondary care centres, principally because of the much lower cost of these devices compared with axial densitometers. They are also very easy to use, requiring minimal set up and positioning, and are relatively portable.
Heel densitometers produce results in terms of BMD (g cm–2) and a heel T-score. Manufacturers' data accompanying devices often quote the WHO criteria indicating that a heel T-score of –2.5 would be indicative of osteoporosis. It is well established, however, that it is inappropriate to interpret any peripheral T-score as one would for axial densitometry [3, 4] and WHO T-score criteria applied to heel DXA would significantly underestimate the prevalence of osteoporosis [3, 5].
In addition, it is well known that measurements of BMD vary from scanner to scanner and between scanner types; particularly between manufacturers because of the variation in acquisition and analysis techniques as well as the absence of a standard calibration technique [6–10]. The International Society for Clinical Densitometry (ISCD) state that although peripheral DXA measurements can theoretically be used to identify patients unlikely to have osteoporosis, and also identify patients who should be treated, this cannot be applied in clinical practice until device-specific cut-off points are established [11].
The National Osteoporosis Society (NOS) has recently produced a revised position statement concerning the recommended use of peripheral densitometry [12]. NOS recommend a triage approach using T-score thresholds, which are specific to each type of peripheral device. These thresholds are defined so that patients with osteoporosis at the hip or spine are identified with 90% sensitivity and 90% specificity. Patients with a peripheral T-score below the lower threshold are very likely to have osteoporosis at the hip or spine, whereas patients with a peripheral T-score above the upper threshold are unlikely to have osteoporosis. Those patients who have a heel T-score that falls between the two thresholds require a hip and spine BMD examination for a definitive diagnosis.
The NOS statement gives device specific threshold data for five peripheral devices, two of which are heel densitometers. Data in this form are not yet published for the Norland Apollo heel densitometer (Cooper Surgical, Trumbull, USA).
In 2001, the Durham and Chester-le-Street Primary Care Trust (PCT) purchased a Norland Apollo DXA scanner for the measurement of heel bone density within the primary care setting. The device was to be operated by the osteoporosis prevention specialist nurse within a protocol agreed between primary and secondary care. The aim of this study was to establish thresholds for Apollo heel BMD T-scores for use in risk stratifying patients within a primary care management protocol.
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Method
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Ethical committee approval was obtained to measure heel BMD with the Apollo in women attending for axial bone density measurements; all fulfilled at least one accepted referral criterion [13]. 76 women aged between 50 years and 75 years (mean age 64.0 years) gave informed written consent. Axial BMD was measured for the lumbar spine, L2–4, and for the non-dominant femur using a Lunar DPX-IQ (General Electric/Lunar Corp., Madison, WI) densitometer. Heel BMD, of the dominant side, was measured with a Norland Apollo portable densitometer. Measurement of the dominant side was acceptable since published data demonstrates no significant difference between BMD measurements of the heel between the right and left sides [5, 14].
Patients were then categorised as either not osteoporotic (T-score > –2.5) or osteoporotic (T-score –2.5), dependent upon their lowest axial T-score, i.e. total femur or spine (L2–4). 23 of these patients were found to have normal axial T-scores, 34 were osteopenic and 19 patients were osteoporotic. Threshold values were initially calculated using this preliminary data so that the Apollo DXA scanner could be used in clinical practice as quickly as possible, although insufficient patients had been examined for the data to achieve the statistical accuracy recommended by NOS, i.e. 95% confidence that the true sensitivity and specificity do not fall below 80%. The initial lower threshold was a T-score of –2.2. The initial upper threshold was 0.1, although this value was obviously very inaccurate since there was a large difference in the T-scores of the patients at the cut off point and those patients immediately above and below them, as is demonstrated in Figure 1
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Figure 1. Scatter plot of heel T-scores for the initial group of patients; 90% of the 57 not osteoporotic patients lie above the lower threshold of –2.2, whilst 90% of the 19 patients who are osteoporotic lie below the higher threshold of 0.1. The limited data points in the osteoporotic group do not allow an accurate cut off value to be selected.
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Patients were seen in a primary care setting by an osteoporosis prevention specialist nurse, who performed heel DXA measurements on patients considered at risk and referred those patients for axial densitometry with heel T-scores below the upper threshold value of 0.1. Heel and axial T-scores were recorded for every patient referred in this way. Data for a further 139 patients between the ages of 50 years and 75 years were recorded, mean age 64.9 years.
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Results
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In total, heel and axial DXA T-scores were measured for 215 women, mean age 64.6 years. Of these, 71 patients were found to be osteoporotic on axial densitometry and 144 patients were not osteoporotic.
Figure 2 shows a scatter plot of heel T-scores for patients in the two diagnostic categories. The line through the not osteoporotic group shows the lower threshold, a T-score of –2.2; 90% of patients who have a T-score above this threshold do not have osteoporosis. The line through the osteoporotic group shows the upper threshold, a T-score of –1.2; 90% of patients who have a T-score below this threshold are osteoporotic.
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Figure 2. Scatter plot of all heel T-scores for patients in the two diagnostic categories; 90% of patients who have a T-score above the lower threshold of –2.2 do not have osteoporosis, whilst 90% of patients who have a T-score below the upper T-score threshold of –1.2 are osteoporotic. Patients with heel T-score between the two threshold values should be referred for axial densitometry.
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Discussion
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The information provided by the manufacturers of the Norland Apollo heel densitometer states that a heel T-score of –2.5 is indicative of osteoporosis. This assumption that the WHO criterion for axial DXA can be applied to heel DXA has been previously demonstrated to be incorrect. Only 15 of the 71 patients (21.1%) who were found to be osteoporotic on axial densitometry had heel T-scores of –2.5.
Data were initially acquired so that a working threshold value could be quickly calculated and used within the primary care setting. These original 76 patients had their hip, spine and heel DXA measurements all performed on the same day. Subsequently, patients were then referred for axial DXA on the basis of established referral criteria or on the value of their heel DXA measurement. These latter results will therefore introduce some bias in the data, as patients who were found to have high T-scores at the heel, above the higher threshold value, were not referred for axial DXA, despite the presence of other risk factors. These patients, with a very low risk of axial osteoporosis, were therefore excluded from the study as it was the intention of the initial threshold values to avoid unnecessary referrals for axial DXA. Since patients with high heel T-scores were excluded, this bias will have the effect of lowering the average T-score measured in the latter group of patients. These patients represent 65% of the total patient group.
This measurement bias prevents these data from being incorporated into the NOS published data for device specific thresholds. Also, NOS consider only women between the ages of 55 years and 70 years, whereas these data have been acquired for patients between 50 years and 75 years of age. However, the acquisition of truly unbiased data within a more narrow age range and with enough patients for statistical accuracy, i.e. 95% confidence that the true sensitivity and specificity do not fall below 80%, would take more than 10 months with this department's current workload. This estimation assumes that only half of all patients referred are within the required age range (based on 2004 data) and less than one third of patients in this age group are likely to be osteoporotic.
These biased data therefore, may be considered an approximation of the true thresholds for the Norland Apollo heel densitometer. The true values are likely be slightly higher, due to the exclusion of patients with high T-scores, therefore caution is advised with heel T-scores just above the upper threshold of –1.2. These patients should be considered to have an equivocal result and be referred for axial DXA to prevent misdiagnosis. Patients with heel T-scores just above the lower threshold (–2.2) will be referred for axial densitometry rather than be given a diagnosis of osteoporosis from the heel DXA alone. In this case, no patients should be misdiagnosed, but the number of referrals to axial DXA will therefore be higher than if the threshold values were unbiased.
The lower threshold of –2.2 was unchanged from that calculated using the initial group of 57 patients. The higher threshold was significantly lowered to –1.2 when a total of 70 osteoporotic patients were considered, compared with the initial value of 0.1 which was calculated from only 19 patients. The difference in these figures is not surprising, and is simply due to the variability of T-scores within small numbers of patients.
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Conclusions
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Thresholds have been calculated for a Norland Apollo heel densitometer; patients with a heel T-score of above –1.2 are very likely to have normal bone density on axial densitometry, whilst patients with heel T-score of below –2.2 are very likely to have osteoporosis at the hip or spine. Patients with heel T-scores that lie between these two thresholds should be referred for axial densitometry. Users of a Norland Apollo heel densitometer may wish to implement these threshold values, with the caveat that they are an approximation but are the best data currently available for this scanner.
Received for publication August 30, 2005.
Revision received October 14, 2005.
Accepted for publication October 31, 2005.
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References
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Abstract
Introduction
