Investigations
1st investigations to order
synovial fluid microscopy, Gram stain, and polarising microscopy
Test
Aspirate synovial fluid for Gram stain and culture before starting antibiotic therapy unless more urgent treatment is indicated.
How to aspirate synovial fluid from the knee and administer intra-articular medication using a medial approach.
How to aspirate synovial fluid from the shoulder and administer intra-articular medication. Video demonstrates a posterior approach to the glenohumeral joint and a lateral approach to the subacromial space.
Send joint aspirate to the microbiology laboratory for urgent processing.[10] In practice, it is important that you chase the results.
Microscopic analysis may reveal the causative organism.[10] A negative result does not exclude the diagnosis of septic arthritis.[10]
Practical tip
Microscopy and Gram stain are not 100% sensitive. Diagnose septic arthritis based on clinical suspicion.[10]
Polarising microscopy may reveal the presence of urate or pyrophosphate crystals (gout or pseudogout).[10] Crystal arthritis and septic arthritis can co-exist.[10]
If you suspect septic arthritis in a prosthetic joint, refer the patient to an orthopaedic surgeon because arthrocentesis should be performed in a sterile operating theatre environment.[10][22]
Result
micro-organisms may be present
urate or pyrophosphate crystals may be present[Figure caption and citation for the preceding image starts]: Calcium pyrophosphate crystals under compensated polarising light microscopyFrom the personal collection of Ann K. Rosenthal, MD [Citation ends].
synovial fluid culture and sensitivities
Test
Aspirate synovial fluid for culture and sensitivities before starting antibiotic therapy unless more urgent treatment is indicated.
How to aspirate synovial fluid from the knee and administer intra-articular medication using a medial approach.
How to aspirate synovial fluid from the shoulder and administer intra-articular medication. Video demonstrates a posterior approach to the glenohumeral joint and a lateral approach to the subacromial space.
Send joint aspirate to the microbiology laboratory for urgent processing.[10]
Microscopic culture may reveal the causative organism and its sensitivity to antibiotics.[10] A negative result does not exclude the diagnosis of septic arthritis.[10]
Synovial fluid culture is positive in:[9]
More than 90% of non-gonococcal arthritis
25% to 70% of patients with gonococcal arthritis
80% of cases of tuberculosis (although this is rare in the UK).
Borrelia burgdorferi cannot be cultured from synovial fluid.
Result
culture may reveal organism type and sensitivities to antibiotic therapy
synovial fluid white cell count
Test
Aspirate synovial fluid for WBC count before starting antibiotic therapy unless more urgent treatment is indicated.
How to aspirate synovial fluid from the knee and administer intra-articular medication using a medial approach.
How to aspirate synovial fluid from the shoulder and administer intra-articular medication. Video demonstrates a posterior approach to the glenohumeral joint and a lateral approach to the subacromial space.
Synovial fluid WBC count is the first result available and while it is neither 100% sensitive nor 100% specific, it is the most useful test in differentiating between septic arthritis and other diagnoses.[28] Non-gonococcal septic arthritis will typically have white cell counts >100,000 per mL and >75% neutrophils compared with other differentials, which will have white cell counts of 2000 to 50,000 and neutrophils <50%. In practice, however, synovial fluid white cell count is not specific enough to accurately differentiate between septic and aseptic inflammation and must be assessed in the clinical context.
A WBC count of >50,000 per mm³ and a polymorphonuclear cell count >90% have been correlated with septic arthritis, but also with crystal arthritis, which can co-exist with septic arthritis.[9][24][28]
If you suspect septic arthritis in a prosthetic joint, refer the patient to an orthopaedic surgeon because arthrocentesis should be performed in a sterile operating theatre environment.[10]
Result
quantity of white cells
blood culture and sensitivities
Test
Draw blood for cultures before starting antibiotic therapy.
Because of haematogenous spread of infection, blood cultures are positive in at least one third of patients with septic arthritis.[10][49]
In some cases the blood culture may be positive in the absence of a positive synovial culture. A negative result does not exclude the diagnosis of septic arthritis.[10]
Result
presence of micro-organisms; subsequent culture revealing organism type and sensitivities to antibiotic therapy
white cell count
Test
Take blood for WBC count as it can help inform a diagnosis, but clinical judgement and results of synovial fluid microscopy are more important.[10] Can help assess bacterial aetiology.[10] Absence of a raised WBC count does not exclude infection in the joint.[10]
Useful in monitoring treatment response.
Result
may be elevated
erythrocyte sedimentation rate (ESR)
Test
Take blood for ESR as it can help inform a diagnosis, but clinical judgement and results of synovial fluid microscopy is more important.[10]
Result
may be elevated
CRP
Test
Take blood for CRP as it can help inform a diagnosis, but clinical judgement and results of synovial fluid microscopy is more important.[10]
Result
elevated
urea and electrolytes
LFTs
Test
Take blood for LFTs to assess for sepsis and end-organ damage as these may influence antibiotic choice.[10]
Result
may be normal or raised
plain x-ray
Investigations to consider
procalcitonin (PCT)
Test
Take blood for PCT as its concentration rises very sharply in the presence of bacterial endotoxin. Serum PCT is a peptide precursor of the hormone calcitonin. In healthy individuals concentration is low (<0.1 ng/mL). Studies in systemic and respiratory infection have suggested that PCT can discriminate between bacterial and non-bacterial inflammation (e.g., rheumatoid arthritis).[26] Small studies have investigated the use of serum PCT in diagnosis of musculoskeletal infection and management.[50][51]
Evidence: Procalcitonin in diagnosing infectious aetiology
There is evidence that procalcitonin levels may support a diagnosis of septic arthritis but should not overrule clinical suspicion.[53]
A systematic review published in August 2017 included 10 studies involving a total of 838 patients and reported an overall sensitivity of serum procalcitonin levels for the diagnosis of septic arthritis of 0.54 (95% CI 0.41 to 0.66) and a specificity of 0.95 (95% CI 0.87 to 0.98).[52] Nine out of 10 studies used procalcitonin cut-off levels of 0.5 ng/mL.
In a more recent study involving 98 patients (18 in the 'gout' group, 26 in the 'calcium pyrophosphate deposition arthritis' group, 16 in the 'mechanical' [osteoarthritis or post-traumatic arthritis] group, 18 in the 'chronic inflammatory rheumatic' group, and 20 in the 'sepsis' group), at a cut-off of 0.5 ng/mL procalcitonin sensitivity was 65% and specificity was 91%. However, the serum procalcitonin levels did not differ between patients with septic or gouty arthritis.[54]
Serum PCT cannot yet be recommended as a routine diagnostic tool.
Serial measurements may indicate response to therapy.[26][53]
MRI
Test
MRI is not routine. Only arrange an MRI if you suspect osteomyelitis.[10]
Result
may show evidence of associated osteomyelitis
synovial fluid polymerase chain reaction (PCR)
Test
PCR for Neisseria gonorrhoeae or Borrelia burgdorferi (Lyme disease) is not routine.[10][45][46]
It may be useful in patients with suspected Lyme disease (alongside serum antibody testing) or suspected gonococcal arthritis, especially if patients present with a migratory pattern of arthralgia, tenosynovial inflammation, and an associated skin rash.[9][14]
Result
may be positive for specific organism
swabs for microscopy, culture, and sensitivity
Test
Take swabs and cultures from any other sources of potential infection identified on history and examination before giving antibiotics, for example:[10]
Pressure sores
Skin lesions
Genitourinary tract (e.g., chronic urinary tract infection, older patients, cervix, urethra, and rectum in sexually active patients)
Respiratory tract (e.g., pharynx in suspected gonococcal arthritis, sore throat).
Result
microscopy may reveal organisms; subsequent culture may identify infection and sensitivities
urine dipstick, microscopy, culture, and sensitivity
Test
Obtain a urine sample in patients with indwelling catheters or recurrent urinary tract infections, as these may be a source for haematogenous spread of infection.
Practical tip
Patients with an indwelling urinary catheter often have a positive dipstick without actual infection. Await culture confirmation before assuming a urinary tract infection is the cause of immobility and confusion in an older person. Be careful not to miss septic arthritis.
Result
may show organisms on microscopy, white cells, blood
subsequent culture may be positive
enzyme-linked immunosorbent assay (ELISA)
Test
If you suspect Lyme arthritis, request an enzyme-linked immunosorbent assay (ELISA) for Borrelia burgdorferi, followed by Western blot if the ELISA is equivocal or positive. See the UK National Institute for Health and Care Excellence guideline on Lyme disease for detailed information on testing. NICE: Lyme disease Opens in new window
Result
may be positive
synovial biopsy
Test
Synovial biopsy for Mycobacterium tuberculosis is positive in about 95% of patients with tuberculosis, and can also identify fungal infections.
Result
may be positive for M tuberculosis or fungi
Emerging tests
calprotectin
Test
Discriminating between septic and aseptic joint inflammation is still a challenge. No test has yet been shown to be clinically useful enough to enter into routine practice. One study on synovial fluid calprotectin levels found that septic arthritis could be discriminated from non-septic inflammatory arthritides, with 76% sensitivity, 94% specificity, and a positive likelihood ratio of 12.2 at the threshold for calprotectin of 150 mg/L.[57] Further research is required to determine its clinical utility.
Result
raised
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