Rapid Diagnostics of Joint Infections Using IS-Pro

Abstract

Diagnosis of bone and joint infections (BJI) relies on microbiological culture which has a long turnaround time and is challenging for certain bacterial species. Rapid molecular methods may alleviate these obstacles. Here, we investigate the diagnostic performance of IS-pro, a broad-scope molecular technique that can detect and identify most bacteria to the species level. IS-pro additionally informs on the amount of human DNA present in a sample, as a measure of leukocyte levels. This test can be performed in 4 h with standard laboratory equipment. Residual material of 591 synovial fluid samples derived from native and prosthetic joints from patients suspected of joint infections that were sent for routine diagnostics was collected and subjected to the IS-pro test. Bacterial species identification as well as bacterial load and human DNA load outcomes of IS-pro were compared to those of culture. At sample level, percent positive agreement (PPA) between IS-pro and culture was 90.6% (95% CI 85.7- to 94%) and negative percent agreement (NPA) was 87.7% (95% CI 84.1 to 90.6%). At species level PPA was 80% (95% CI 74.3 to 84.7%). IS-pro yielded 83 extra bacterial detections over culture for which we found supporting evidence for true positivity in 40% of the extra detections. Missed detections by IS-pro were mostly related to common skin species in low abundance. Bacterial and human DNA signals measured by IS-pro were comparable to bacterial loads and leukocyte counts reported by routine diagnostics. We conclude that IS-pro showed an excellent performance for fast diagnostics of bacterial BJI.

Keywords: IS-pro; bacteria; joint infections; rapid diagnostics.

FIG 1

Additional detections by IS-pro and culture with corroborating evidence for true positivity. (A) Signals of all additional IS-pro detections per species. (B) All additional detections by culture grouped per species and per culture load. (A) and (B) Colors indicate additional clinical evidence: in red: the species detected is present in culture in another sample from the same subject; in yellow: leukocyte levels fall into the category “many”; in blue: both of the categories red and yellow apply; in gray: none of the above apply. Crosses indicate samples that were positive by S. aureus qPCR. CNS: all non-S. aureus Staphylococci detections combined.

FIG 2

Bacterial load comparison between culture and IS-pro. IS-pro signals are indicated as fluorescence (in relative fluorescence units [RFU]) of the PCR products of the detected species. Culture loads are given in categories as explained in the Methods section. P values calculated by the Mann-Whitney U test are indicated between the compared categories. Shown are all concordant detections.

FIG 3

Box plot showing human DNA signal intensities measured by IS-pro in RFU in relation to leukocyte levels reported by routine diagnostics. In the bottom table, the top row indicates the leukocyte levels as reported by routine diagnostics. The bottom rows indicate the total number of samples (count), the calculated median RFU and the number of outliers. Leukocyte levels were not reported for 83 samples. P values calculated by the Mann-Whitney U test are indicated between the compared categories.