Current issues of ACP Journal Club are published in Annals of Internal Medicine


Diagnosis

Most tests had a low sensitivity and high specificity for predicting osteomyelitis in diabetic foot ulcers

ACP J Club. 1992 Jan-Feb;116:18. doi:10.7326/ACPJC-1992-116-1-018


Source Citation

Newman LG, Waller J, Palestro CJ, et al. Unsuspected osteomyelitis in diabetic foot ulcers. Diagnosis and monitoring by leukocyte scanning with indium In 111 oxyquinoline. JAMA. 1991 Sep 4;266:1246-51.


Abstract

Objective

To determine the prevalence of and to evaluate diagnostic tests for osteomyelitis in diabetic foot ulcers.

Design

Comparison of clinical findings, roentgenograms, leukocyte scans, and bone scans with the diagnostic standard, bone histologic and culture findings, in the diagnosis of osteomyelitis.

Setting

Tertiary care medical center.

Patients

54 diabetic patients were referred for study. 35 patients (mean age, 55 years) with 41 foot ulcers were included. Patients who had been taking antibiotics for > 7 days, had severe peripheral vascular disease, or had an incomplete bone biopsy were excluded. The median ulcer duration was 4 months. 61% of patients had had a full or partial foot amputation.

Description of tests and diagnostic standard

Before a bone biopsy was done, each patient had roentgenograms (3 views), a leukocyte scan, and a bone scan. Clinical findings, ulcer characteristics, and laboratory tests were also assessed as diagnostic tests for osteomyelitis. Bone biopsies were obtained by surgical debridement (n = 4) or amputation (n = 5), or by a 15-gauge trocar placed through a 5-mm incision in an area noncontinuous with the foot ulcer (n = 32). The diagnostic standard for osteomyelitis was a positive bone culture or abnormal histologic findings. The radiologist, pathologist, and nuclear medicine physician interpreted studies without knowledge of other test results.

Main outcome measures

Sensitivity and specificity of clinical findings, bone scans, leukocyte scans, and roentgenograms were calculated for the diagnosis of osteomyelitis.

Main results

Osteomyelitis was found underlying 28 (68%) of 41 foot ulcers. Only 9 (32%) of these 28 cases were suspected clinically. Sensitivity, specificity, and likelihood ratio of a positive test (+LR) and negative test (-LR) for roentgenograms were 28%, 92%, {4, and 0.8}*; for bone scans, 69%, 39%, {1.1, and 0.8}*; and for 24-hour leukocyte scans, 89%, 69%, {2.9, and 0.2}.*

The best clinical marker of osteomyelitis was an ulcer area > 2 cm2, which had a sensitivity of 56%, specificity of 92%,{+LR of 7, and -LR of 0.5}.* Bone exposed within the ulcer, an erythrocyte sedimentation rate > 100 mm/h, and a clinical suspicion of osteomyelitis each had perfect specificity (100% positive predictive value), low sensitivity (32%, 23%, and 32%, and {-LR (0.7, 0.8, and 0.7, respectively}).*

Conclusions

Osteomyelitis underlying diabetic foot ulcers often occured without clinical suspicion or signs of inflammation. The 24-hour leukocyte scan was a promising noninvasive diagnostic test.

Source of funding: No external funding.

Address for article reprint: Dr. L.G. Newman, Mount Sinai Medical Center, Box 1087, One Gustave L. Levy Place, New York, NY 10029, USA.

*Numbers calculated from data in article.


Commentary

This study suggests that osteomyelitis underlying diabetic foot ulcers is often undiagnosed and therefore inadequately treated. An alarming 33% of shallow ulcers in this series had an underlying osteomyelitis. However, primary care physicians should be cautious in embracing these study results because their patients may differ from those described by Newman and colleagues. 61% of study subjects had previous amputations and probably had more severe complications of diabetes than patients in primary care settings.

Although bone exposure, an erythrocyte sedimentation rate of > 100 mm/h, and clinical suspicion are highly specific markers of osteomyelitis, the low sensitivities compare unfavorably with leukocyte scanning and bone biopsy. This problem might be overcome if the 3 tests are combined, but the study did not assess this.

The test characteristics of the leukocyte scan appear to be excellent; however, confirmation is needed before this test alone becomes the standard for diagnosis. The sample size was small and the confidence intervals around the sensitivity and specificity were very wide. Although the sensitivity and specificity of the leukocyte scan were higher than those of the bone scan, because the confidence intervals overlapped, we cannot be certain that the leukocyte scan is the better test. Even if the test properties of the leukocyte scan are superior, whether use of the test improves outcome still needs to be evaluated.

A bone biopsy may still be warranted to confirm the diagnosis. However, this diagnostic standard may also be problematic, given the possibility of false positives because of bacterial contamination from adjacent soft-tissue infection. The latter problem can be minimized by sampling tissue using sterile technique in an area non-contiguous with the foot ulcer, as was done in this study. In practice, bone biopsy has not usually been done to identify the pathogenic organism, but increasing bacterial resistance to the broad-spectrum antibiotics often prescribed empirically may now make bone biopsy and culture important for the diagnosis and treatment of osteomyelitis.

Arthur T. Evans, MD, MPH
University of North CarolinaChapel Hill, North Carolina, USA