1556 THE JOURNAL OF BONE AND JOINT SURGERY  CHILDREN’S ORTHOPAEDICS The use of CRP within a clinical prediction algorithm for the differentiation of septic arthritis and transient synovitis in children R. Singhal, D. C. Perry, F. N. Khan, D. Cohen, H. L. Stevenson, L. A. James, J. S. Sampath, C. E. Bruce From Alder Hey Children’s Hospital, Liverpool, United Kingdom  R. Singhal, MBBS, MS (Ortho), MRCSEd, Registrar  D. C. Perry, MB, ChB (Hons), MRCS (Eng), Registrar, PhD Student  F. N. Khan, MBBS, MRCSEd, Specialty Trainee  D. Cohen, MBChB, FRCS (Tr & Orth), Registrar  H. L. Stevenson, MBCh, FRCS (Tr & Orth), Registrar  L. A. James, MBBS, FRCSEd (Tr & Orth), Consultant Orthopaedic Surgeon  C. E. Bruce, MBChB, FRCS (Tr & Orth), Consultant Orthopaedic Surgeon Alder Hey Children’s Hospital, Department of Orthopaedic Surgery, Eaton Road, West Derby, Liverpool L12 2AP, UK.  J. S. Sampath, MBBS, MSc, FRCSEd (Tr & Orth), Consultant Orthopaedic Surgeon SPARSH Hospital, Department of Orthopaedics, The Health City, Bommasandra Industrial Area, Hosur Road, Bangalore 560099, India. Correspondence should be sent to Mr R. Singhal; e-mail: singhal.rohit75@gmail.com ©2011 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.93B11. 26857 $2.00 J Bone Joint Surg Br 2011;93-B:1556–61. Received 3 February 2011; Accepted after revision 15 July 2011 Clinical prediction algorithms are used to differentiate transient synovitis from septic arthritis. These algorithms typically include the erythrocyte sedimentation rate (ESR), although in clinical practice measurement of the C-reactive protein (CRP) has largely replaced the ESR. We evaluated the use of CRP in a predictive algorithm. The records of 311 children with an effusion of the hip, which was confirmed on ultrasound, were reviewed (mean age 5.3 years (0.2 to 15.1)). Of these, 269 resolved without intervention and without long-term sequelae and were considered to have had transient synovitis. The remaining 42 underwent arthrotomy because of suspicion of septic arthritis. Infection was confirmed in 29 (18 had micro-organisms isolated and 11 had a high synovial fluid white cell count). In the remaining 13 no evidence of infection was found and they were also considered to have had transient synovitis. In total 29 hips were categorised as septic arthritis and 282 as transient synovitis. The temperature, weight-bearing status, peripheral white blood cell count and CRP was reviewed in each patient. A CRP > 20 mg/l was the strongest independent risk factor for septic arthritis (odds ratio 81.9, p < 0.001). A multivariable prediction model revealed that only two determinants (weight-bearing status and CRP > 20 mg/l) were independent in differentiating septic arthritis from transient synovitis. Individuals with neither predictor had a < 1% probability of septic arthritis, but those with both had a 74% probability of septic arthritis. A two-variable algorithm can therefore quantify the risk of septic arthritis, and is an excellent negative predictor. Several algorithms have been proposed to assist in differentiating between septic arthritis and transient synovitis of the hip in children. 1-6 This distinction is crucial as the natural history, and hence treatment, of the two diseases differs greatly. Whilst transient synovitis is a benign condition with little potential for long-term serious damage, septic arthritis can lead to a poor outcome, particularly if the treatment is delayed. 7 Kocher, Zurakowski and Kasser 1 initially described several variables to aid in the differ- entiation of these conditions: temperature > 38.5°C, inability to weight-bear, erythro- cyte sedimentation rate (ESR) > 40 mm/hr and peripheral white blood cell count > 12 000 cells/mm 3 . However, in these condi- tions ESR has largely been replaced by estima- tion of the C-Reactive Protein (CRP). 8 CRP is an acute phase reactant, which has a more rapid response to disease than ESR. It is influenced less by external factors and its measurement requires a smaller blood volume. In a further validation study by Kocher et al 3 it was acknowledged that CRP had benefits over ESR, yet they were unable to include this parameter owing to their lack of necessary data. Whilst some studies have included CRP in prediction models this has been in combina- tion with ESR. 4,6 In our centre we frequently make decisions in differentiating transient syn- ovitis and septic arthritis based on the use of CRP without consideration of ESR. This study was designed to assess the use of CRP in a clinical prediction algorithm to dif- ferentiate transient synovitis and septic arthri- tis, based on the framework suggested by Kocher et al. 3 Patients and Methods A review was undertaken of the records of all patients under the age of 16 years presenting with an acute, new-onset, atraumatic limp or hip pain between January 2004 and October 2009. All patients underwent an ultrasound evaluation of the hip as part of a protocol to investigate the ‘irritable hip’. Neonates were excluded. Demographic data, clinical examination, haematological parameters and ultrasound