Tests GPs Should Consider When Diagnosing Inflammatory ArthritisLaboratory tests are a useful adjunct to the history and physical examination of a patient with musculoskeletal complaints. Appropriate use of the tests aids in diagnosing and ruling out specific disease entities. Inappropriate use can lead to misdiagnosis and unnecessary anxiety for the patient. The cornerstone of diagnosing inflammatory arthritis rests in the history and physical examination. Inflammatory arthritis should be suspected in a patient of any age if they present with a history of joint swelling and morning stiffness lasting > 45 minutes. Degenerative arthritis (osteoarthritis) will typically present as pain and stiffness, sometimes referred to as "gel phenomenon", but the stiffness eases after a few a few minutes of movement. Routine investigations should include a CBC, Creatinine, liver function tests and a urinalysis. A normochromic, normocytic anemia in the setting of a thrombocytosis would be in keeping with inflammatory arthritis, and highlights the systemic nature of the disease. If liver function tests are elevated, drugs, viruses or autoimmune hepatitis should be suspected and appropriate investigations arranged. The urinalysis should be reviewed for proteinuria and hematuria and the presence of red cell casts. Acute phase reactants are liver-manufactured plasma proteins that increase following inflammation and tissue necrosis. Coagulation proteins, transport proteins and complement components are acute phase reactants, but the two most frequently used tests are the ESR and C Reactive Protein (CRP). The ESR is a rapid and inexpensive test, but it is slow to change and subject to artifact. Many factors influence the ESR, including age and sex. A useful equation for estimating the upper limit of normal is:
Bear in mind however that the ESR can be elevated or depressed by many factors. As with all tests, the results must be interpreted in the clinical context in which it was ordered. (Table 1).
Perhaps no test causes more confusion than the rheumatoid factor test. It must be remembered that it is a test, not a diagnosis. It is a non-specific protein commonly found in rheumatologic and non-rheumatologic conditions. The prevalence in patients with rheumatoid arthritis increases with disease duration, and may be negative early in the course of disease. For example, at 3 months, 33% patients with RA will be rheumatoid factor positive, at 6 months, 60% positive, and at 12 months, 75% positive (Table 2). RF may also be elevated in up to 20% of first degree relatives of patients with RA. Do order the test in the patient with prolonged morning stiffness and swollen joints, but do not request it in the work-up of a patient with low back pain (RA does not affect the low back), nor in the patient with classic osteoarthritis. Antinuclear antibodies (ANA) are frequently found in the normal population despite being considered a hallmark of connective tissue diseases. They can be found in acute viral infections, in chronic infections, malignancies and they can be drug induced. Although the test is non-specific, a negative ANA virtually excludes systemic lupus as a diagnosis. A positive ANA must always be interpreted in the clinical context. New onset Raynaud's phenomenon, muscle weakness, swollen joints or photosensitive rash should prompt referral to a rheumatologist, who would then order the appropriate specific autoantibodies (Table 3). The investigation of a patient with low back pain should not include ANA, as none of the connective tissue diseases associated with ANA are associated with inflammatory changes in the lumbar spine or sacroiliac joints.
If the ANA and RF are unhelpful in the investigation of low back pain, what tests are? Mechanical low back pain will not be accompanied by any laboratory abnormalities, whereas inflammatory markers such as ESR and CRP frequently will characterize inflammatory back pain. Radiographs of the sacroiliac joints would be the next most appropriate test, not more bloodwork, particularly not HLA B27. This genetic marker is present in a significant percentage of the population (1-50% dependent on racial group). Only 2-20% of unselected HLA-B27 positive individuals will have spondylitis, which leads to both over-diagnosis in patients positive for the gene and under-diagnosis in those negative for the gene. For example, only 60-70% patients with inflammatory bowel disease and spondylitis are HLA -B27 positive. If one excluded spondylitis on the basis of a negative B27 test, one would have missed 30% of those affected. Similarly, one would have over-diagnosed the disease in 80% or more of those positive for the gene. The diagnosis should be determined on clinical grounds of back pain associated with morning stiffness, decreased lumbar spine movements, positive markers of inflammation and the presence of erosive disease in the SI joints on radiographs. Whenever someone is faced with a patient with swollen joints it is reasonable to consider diagnostic joint aspiration. This is an easy procedure to do if the knees are involved. The fluid should be sent for the "3 C's": cells, crystals and culture. A common mistake is sending the fluid in a single container: a standard sterile container will do for the culture, but for cell and crystal analysis, approximately 5 ml fluid should be sent in an EDTA container (standard lavender-stoppered tube). The number of white cells is helpful in determining whether the fluid is inflammatory or non-inflammatory: Osteoarthritis will typically have more white cells than normal fluid but not as many as a rheumatoid, gouty or septic joint. (Table 4).
Gout typically presents initially as a monoarthritis, but after repeat attacks, polyarticular involvement can occur. There is no blood test that is helpful in this regard: serum uric acid may be normal during an acute attack: the most helpful test is the synovial fluid analysis confirming the presence of urate crystals. Serum uric acid should be monitored when a decision is made to introduce urate lowering drugs, which cost effectiveness studies suggest should occur after the second attack of gout (Table 3). In summary, the approach to the patient with joint complaints should start with a history and physical examination. If the history is compatible with an inflammatory joint disorder, then consider what the most likely diagnosis is given the patient's age and sex. Don't hesitate to refer the patient to a rheumatologist when in doubt, and particularly when an inflammatory arthropathy is strongly suggested by the test results, as prompt treatment with the appropriate agent may limit the development of irreversible erosive damage which can occur within 6 months of onset of symptoms. Thanks to Dr. Imtlaz Khan, Rheumatologist on staff at the Trillium Helath Centre in Mississauga, Ontario for reviewing the draft copy of this article. References and Recommended Reading:
Return to Archives Page ] [ Berries Home Page |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||