Can synovial fluid white blood cell count predict shoulder periprosthetic infection?

Defining a Synovial Fluid White Blood Cell Count Threshold to Predict Periprosthetic Infection after Shoulder Arthroplasty

These authors sought to define a threshold for synovial fluid white blood cell count (WBC) and the reliability of microbiological cultures in predicting shoulder periprosthetic infection (PJI).

They conducted a retrospective study of preoperative and intraoperative fluid aspiration of 31 patients who underwent a revision of a shoulder arthroplasty (15 with PSI defined by the Infectious Diseases Society of America (IDSA) criteria (see this link) and 16 without infection).

WBC was significantly higher in patients with PSI than in other patients. A threshold of 2800 leucocytes/mm3 showed a sensitivity of 87% and a specificity of 88% (AUROC 0.92). 

Microbiological cultures showed a sensitivity of 76% and a specificity of 100%. The observation that almost one quarter of the joint aspirate cultures were negative in cases of periprosthetic infection (PJI) indicates that positive cultures of joint fluid can "rule in" a PJI but a negative culture of joint fluid cannot "rule out" a PJI.

Interestingly, joint aspirate culture was less likely to grow Cutibacterium (23%) than the tissue cultures obtained at revision surgery (40%) (see below). There may be at least two reasons for this finding (1) the joint aspirate provides a single specimen whereas multiple (at least three) tissue samples increase the chances of recovering the organism and (2) Cutibacterium is more commonly present in a biofilm on tissue and implants than in a planktonic form where it would be accessible in a fluid sample. 

Comment: Even though the number of cases in this study is small, the results are of interest and need to be confirmed in larger cohorts. This should be straightforward, in that white blood cell counts and cultures are easy to obtain and relatively inexpensive. 

One issue not addressed in this study is the rate of insufficient sample volume, that is the percentage of cases coming for revision in which the amount of fluid obtained is not enough for both a WBC count and culture. How should this result be considered? 

A second issue is that the utility of these tests should be examined in terms of the "pre-test probability". It is known that Cutibacterium PJI can present on one hand with systemic signs, increased blood markers (CRP, ESR, WBC) and on the other with only unexplained pain and stiffness.  In the former the pre test probability of PJI is high and in the latter less so. How diagnostic are joint fluid aspirate WBC counts and cultures in these two groups?

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Here are some videos that are of shoulder interest

Shoulder arthritis - what you need to know (see this link)

Shoulder arthritis - x-ray appearance (see this link)

The smooth and move for irreparable cuff tears (see this link)

The total shoulder arthroplasty (see this link).

The ream and run technique is shown in this link.

The cuff tear arthropathy arthroplasty (see this link).

The reverse total shoulder arthroplasty (see this link).