Handling of Doubtful WBC Scintigraphies in Patients with Suspected Prosthetic Joint Infections

Abstract

Despite the application of EANM recommendations for radiolabelled white-blood-cells (WBC) scintigraphy, some cases still remain doubtful based only on visual analysis. The aim of this study was to investigate the role of semi-quantitative analysis and bone marrow scan (BMS) in solving doubtful cases. We retrospectively evaluated all [^99mTc]HMPAO-WBC scintigraphies performed, in the last 7 years, for a suspected monolateral prosthetic joint infection (PJI). In doubtful cases, we used five different thresholds of increase of target-to-background (T/B) ratio, between delayed and late images, as criteria of positivity (5%, 10%, 15%, 20% and 30%). BMS were also analysed and sensitivity, specificity and accuracy of different methods were calculated according to final diagnosis. The sensitivity, specificity and accuracy were, respectively, 77.8%, 43.8% and 53.0% for the cut-off at 5%; 72.2%, 66.7% and 68.2% for the cut-off at 10%; 66.7%, 75.0% and 72.7% for the cut-off at 15%; 66.7%, 85.4% and 80.3% for the cut-off at 20%; 33.3%, 93.8% and 77.3% for the cut-off at 30%. BMS provided a significantly higher diagnostic performance than 5%, 10% and 15% thresholds. Conversely, we did not observe any statistically significant difference between BMS and the cut-off of more than 20%. Therefore, doubtful cases should be analysed semi-quantitatively. An increase in T/B ratio of more than 20% between delayed and late images, should be considered as a criterion of positivity, thus avoiding BMS.

Keywords: BMS; WBC scintigraphy; qualitative analysis; semi-quantitative analysis.

Figure 1

Anterior views acquired 3 h and 20 h after i.v. injection of radiolabelled leukocytes, with times corrected for isotope decay and displayed by using absolute counts and the same intensity colour scale. Images show an example of the methodology used for drawing regions of interests (ROIs) for all the criteria (A–F). In all doubtful cases, except for criterion B, irregular ROIs were automatically drawn on the target (T) of late images by fixing a threshold of 40% of maximum pixel, mirrored on the contralateral background (B) and copied on T and B of delayed images. For criterion B, due to the presence of bone marrow interference on the evaluation of the suspected area, ROIs were manually drawn on the whole prosthesis.

Figure 2

Flowchart of patients included in the study.

Figure 3

These images show an example of false negative BMS in a patient with histologically confirmed infection of right hip prosthesis. From the left to right: the uptake on the right trochanter at 3 h does not change after 20 h at visual analysis therefore her WBC scintigraphy was classified as doubtful according to criterion “A”. However, we observed an increase of more than 20% at semi-quantitative analysis. The patient performed a bone marrow scan (BMS) (image on the right panel) that showed a congruent uptake with WBC thus excluding the infection. After surgery, an infection by St. Aureus was detected.

Figure 4

Proposed flow-chart for doubtful qualitative analysis of WBC scintigraphy.