Improved method for detecting knee replacement infections based on extended combined 99mTc-white blood cell/bone imaging.

The purpose of this study was to evaluate whether an extension of the imaging time to 24 h post-injection improves the diagnostic accuracy of technetium-99m-hexamethylpropyleneamine oxime (99mTc-HMPAO) leucocyte imaging in detecting knee replacement infections. Thirty patients were studied, with infection confirmed in eight (27%) and excluded in 22 on the basis of clinical and microbiological findings. Leucocyte imaging was carried out at 2-4 h (routine images) and at 24 h (late images) post-injection. For comparison, bone imaging with technetium-99m-hydroxydiphosphonate (99mTc-HDP) was carried out at arterial, soft tissue and metabolic phases. Late leucocyte imaging was found to be more sensitive (100% vs. 87.5%) and more specific (82% vs. 77%) than routine leucocyte imaging in detecting infections. All the bone imaging methods showed a sensitivity of 100%, whereas the specificity varied from only 5% to 23%. All procedures had high negative predictive values (NPVs) (94 to 100%) for excluding infection. However, the positive predictive value (PPV) was only 28 to 32% for bone imaging and 58% for routine leucocyte imaging, whereas late leucocyte imaging showed a PPV of 67% and a diagnostic accuracy of 87%. The data indicate that late leucocyte imaging may be superior to routine leucocyte imaging for examining patients with symptomatic knee replacements.