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2112. Determining the Significance of a “Dry Tap”: The Outcome of Hip Joint Aspiration and the Rate of Infection in Patients with a Dry Tap
Authors * Denotes Presenting Author
  1. Aline Serfaty *; NYU Langone Medical Center/NYU Orthopedic Hospital
  2. Adam Jacobs; NYU Langone Medical Center/NYU Orthopedic Hospital
  3. Soterios Gyftopoulos; NYU Langone Medical Center/NYU Orthopedic Hospital
  4. Mohammad Samim; NYU Langone Medical Center/NYU Orthopedic Hospital
Objective:
To determine the rate of infection in patients with suspected hip septic arthritis who underwent image-guided hip aspiration that resulted in dry tap.

Materials and Methods:
A retrospective PACS query between 2010-2020 identified subjects who underwent image-guided hip aspiration – both native hip (NH) and total hip arthroplasty (THA) – for suspected septic arthritis yielding no native fluid with subsequent lavage and re-aspiration. Medical records were reviewed to determine the diagnosis of infection. Musculoskeletal Infection Society criteria were used for establishing a periprosthetic joint infection (PJI) diagnosis. ESR and CRP values, peri-aspiration antibiotic treatment, presence and grade of joint effusion (JE) on peri-aspiration MRI or CT, image guidance modality, lavage and re-aspiration volumes, and joint aspirate cell count and culture analysis were recorded. Pseudocapsule disruption with fluid outside the joint (PD) and presence of sinus tract (ST) were recorded in THA patients with MRI. Statistical analysis included Mann-Whitney and Fisher exact tests and multivariable logistic regression.

Results:
Out of 982 aspirations, 215 (208 patients, mean age of 60.2 years) met the inclusion criteria, with 30 (14%) joints deemed infected and 185 (86%) not infected. Among 71/215 (33.0%) NH, 7 (9.9%) were infected and 64 (90.1%) were not infected, and among 144/215 (66.9%) THA, 23 (16%) were infected and 121 (84%) were not infected. ESR (p=0.042) and CRP (p=0.015) were elevated in 21 (80.8%) and 22 (81.5%) patients, respectively, in the infected group and in 64 (57.7%) and 60 (55.6%) in the not infected group. JE was present in 11 (57.9%) infected and 17 (25.0%) not infected patients (p=0.011). Aspiration culture was positive in 12/30 (40%) infected hips, 3/7 (42.8%) infected NH, and 9/23 (39.1%) infected THA. Among infected patients, there was no significant association between positive aspiration culture or cell count and presence/grade of JE, image guidance modality, or lavage/re-aspirate volume. Of THA patients who had a peri-aspiration MRI, 8/12 (66.7%) had PD and 7/12 (58.3%) had ST in the infected group vs 5/42 (11.9%) and 0/42 in the not infected group (both p<0.001). Among THA patients who had re-aspiration cell count analysis, PMN>80% was present in 8/9 (88.9%) infected compared to 4/28 (14.3%) not infected patients (p=0.001). Multivariable logistic regression showed that among THA patients, PD (p=0.005) and joint fluid (p=0.042) were significant independent predictors of infection (AUC of 0.83), as were elevated CRP (p=0.044) and joint fluid (p=0.017) (AUC of 0.85).

Conclusion:
Among patients who underwent image-guided hip aspiration resulting in a dry tap, 85-90% were not infected. Culture from lavage was positive in 40% of infected hips. In THA patients suspected for PJI, pre-aspiration MRI can be helpful to determine presence of joint fluid or pseudocapsule disruption with fluid outside the joint which, along with elevated CRP, were significant independent predictors of infection and can guide the target of aspiration.