How Do You Make a Diagnosis of an Infected Arthroplasty?

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Chapter 91 How Do You Make a Diagnosis of an Infected Arthroplasty?

ALEXANDER. SIEGMETH, MD, FRCS (TR & ORTH), DONALD S. GARBUZ, MD, MHSC, FRCSC, BASSAM A. MASRI, MD, FRCSC

Infection after a total hip or knee arthroplasty is an uncommon but serious complication with significant impact. Although the postoperative infection rate has decreased from 9% to 1% to 2%, large centers still treat a significant number of patients with infected arthroplasties every year. With the aging population and also younger patients demanding joint replacement procedures, the problem is likely to increase over the years to come.

Rational methods to prevent and diagnose infection must be established. Diagnosing an infection after a joint replacement can be straightforward in the acute postoperative period but challenging if it occurs months or years later. Accurate diagnosis is therefore important to avoid erroneous surgery.

Aseptic loosening is currently the most common diagnosis leading to a revision arthroplasty. However, infection must be excluded in all cases. The consequences of failing to correctly diagnose an infection will have serious consequences on outcome of the revision procedure, with reinfection occurring in most cases. The purpose of this chapter is to outline the various diagnostic options and to evaluate their accuracy based on the best evidence available in the literature.

OPTIONS

History and clinical examination are an important first step in diagnosing a prosthetic infection. Preoperative investigations include blood tests, aspiration, radiographs, and nuclear medicine techniques. Intraoperative tests include analysis of synovial fluid, Gram staining, histologic analysis of a frozen section, or intraoperative cultures.

EVIDENCE

History and Clinical Examination

No study has investigated solely the sensitivity and specificity of history and clinical examination to accurately diagnose a prosthetic infection. However, Spangehl and colleagues ¹ correctly diagnosed 27 of 35 infected total hip arthroplasties with history and examination alone (Level I). All 35 infections were later confirmed with positive intraoperative cultures. Acute onset of pain, systemic illness, and sinus formation were the most commonly found signs and symptoms. No studies comment on the value of history and examination in correctly diagnosing an infected knee arthroplasty. History and physical examination are useful because they give you an initial index of suspicion or a pretest likelihood of infection. Further diagnostic testing simply changes the pretest likelihood of disease. As such, a patient who presents with a low likelihood of infection based on history and physical examination, will require much more stringent criteria to clearly diagnose infection. However, a patient who presents with florid signs and symptoms of infection will require much less stringent criteria for the confirmation of infection. This is an application of Bayes’ theorem in probability theory, and this is why it is important to understand how to apply the various tests that are discussed in the context of the initial encounter with the patient, namely, the history and physical examination.

Preoperative Hematologic Tests

White Blood Cell Count.

Obtaining a white blood cell count (WBC) is a routine preoperative investigation that is of little diagnostic value. In a prospective study (Level I evidence), Spangehl and colleagues ¹ evaluated the diagnostic accuracy to correctly diagnose an infection in 202 hips. With a value of more than 11.0 = 10⁹ WBC/L considered to be a positive result indicating infection, they found a sensitivity of 0.20 and a specificity of 0.96. The positive predictive value, however, was only 0.54, and the negative predictive value 0.85. Di Cesare and coworkers² (Level IV) undertook a prospective, case–control study of 58 patients undergoing reoperation. Seventeen were diagnosed as having a prosthetic infection. The sensitivity of WBCs was 0.47, and the specificity was 1.00. The positive predictive value was 1.00, and the negative predictive value was 0.82. In a retrospective case series (Level IV evidence), Canner and coauthors³ note a low prevalence of increased WBC counts in patients with an infected total joint arthroplasty. Therefore, based on the evidence, WBC count is of limited use, except when the pretest probability is high.

Erythrocyte Sedimentation Rate and C-Reactive Protein.

The erythrocyte sedimentation rate (ESR) is a measure of erythrocyte rouleaux formation, which occurs whenever an inflammatory condition is present. C-reactive protein (CRP) is an acute-phase protein synthesized in the liver. As with all acute-phase reactants, CRP level is increased in many inflammatory, infectious, and some neoplastic conditions. Spangehl and colleagues ¹ (Level I) prospectively analyzed the ESR and CRP level in 171 and 142 revision hip arthroplasties. ESR had a sensitivity of 0.82, specificity of 0.85, positive predictive value of 0.58, and negative predictive value of 0.95. CRP had a sensitivity of 0.96, specificity of 0.92, positive predictive value of 0.74, and negative predictive value of 0.99. If both tests were negative (ESR <30 mm/hr, CRP <10 mg/L), the probability of infection was zero. When both tests were positive, the probability of infection was 0.83. Virolainen and researchers⁴ (Level IV) also conclude that combined ESR and CRP are of value in the preoperative evaluation. For ESR, Di Cesare and coworkers² (Level IV) found a sensitivity of 1.0 and specificity of 0.56. CRP had a sensitivity of 0.95 and specificity of 0.76.

Greidanus and investigators ⁵ (Level I) evaluated 145 patients presenting for revision total knee arthroplasty for the presence of infection using the ESR and CRP. The ESR and CRP were obtained at the time of clinical assessment before definitive revision total knee arthroplasty. All patients had undergone preoperative aspiration for culture and had intraoperative periprosthetic tissue sent for bacterial culture. A diagnosis of infection was established for 45 of 151 knees that underwent revision total knee arthroplasty (prevalence, 0.298). The ESR (sensitivity, 0.93; specificity, 0.83; likelihood ratio positive, 5.81; accuracy, 0.86) and CRP (sensitivity, 0.91, specificity, 0.86; likelihood ratio positive, 6.89; accuracy, 0.86) had excellent diagnostic test performance. Combination testing of ESR and CRP together increases overall sensitivity to 0.95 for the diagnosis of infection. In this study, using receiver operating characteristics curves, the optimal cut point for ESR was found to be 22.5 mm/hr, and that of CRP was found to be 13.5 mg/L.

Serum Interleukin-6.

Interleukin-6 (IL-6) is a factor produced by monocytes and macrophages. It functions as a hepatocyte-stimulating factor and induces the production of acute-phase reactants. Wirtz and coworkers ⁶ established that it returns to normal after total joint arthroplasties. Di Cesare and coworkers² (Level III) selected 58 patients who underwent a revision procedure and measured the IL-6 levels before and after surgery. Seventeen of the 58 patients were infected as determined after surgery with intraoperative cultures. The sensitivity of IL-6 was 1.0, the specificity was 0.95, the positive predictive value was 0.89, and the negative predictive value was 1.00. However, this study was done on a small and selected sample, and further large studies (Levels I and II) are needed to determine the value of IL-6 in accurately detecting infection before surgery.