Laboratory Studies

A complete cell count (CBC) with differential, urinalysis, and renal and liver function tests should be performed if asymptomatic rheumatic disease is suspected. Importantly, the frequency of abnormal laboratory results increases with increasing age in the normal population, even in the absence of disease, including common tests such as erythrocyte sedimentation rate (ESR), uric acid, antinuclear antibodies (ANAs), and rheumatoid factor (RF). Thus, arthritis panels can confuse the situation and should not be performed routinely. For example, only 80% of patients with RA have a positive RF. RF is a serum autoantibody against immunoglobulin G (IgG). Up to 4% of the healthy population has a positive RF, which is also frequently positive in patients with chronic obstructive pulmonary disease (COPD), viral hepatitis, and sarcoidosis, and can also be positive in malignancy, and primary biliary cirrhosis and other autoimmune diseases. The higher the RF titer, however, the more likely it is caused by RA. ANA test results are positive in 95% of patients with lupus, and the test is often used to screen for SLE, but the result is also positive in 5% of the normal population. Drug use, age, and other factors might also cause a positive ANA test result. ANA titer also does not correlate exactly with changes in disease activity, so it should not be ordered in the absence of systemic symptoms. A patient with a positive ANA without clinical features is unlikely to have SLE. However, higher titers of ANA make it more likely that the result is related to lupus or another rheumatologic disorder.

Laboratory studies can be helpful in monitoring disease activity and drug toxicity as well as in establishing a diagnosis. CBC can detect anemia secondary to the chronic disease of RA or from NSAID-induced gastrointestinal (GI) blood loss. Patients with SLE can have hemolytic anemia, throm-bocytopenia, or lymphopenia. Urinalysis can detect renal disease secondary to SLE, NSAIDs, or disease-modifying antirheumatic drugs (DMARDs) being used to treat RA. An elevated uric acid level can suggest gout. Acute-phase reac-tants such as ESR and C-reactive protein (CRP) can be useful to monitor disease activity but are nonspecific; they can also be negative in the presence of active disease. Patients with temporal arteritis and polymyalgia rheumatica almost always have a greatly elevated ESR. With weakness or muscle pain, CK level should be measured and arthralgias with abnormal liver enzyme levels followed up with hepatitis viral serologies.

Other tests, such as HLA-B27, antineutrophil cytoplasmic antibody, Lyme or parvovirus serologies, myositis-specific antibodies (anti-Jo-1), and antiphospholipid antibodies, are useful only when the clinical suspicion is high for spondyloarthropathies, Wegener's granulomatosis, Lyme or parvovirus infection, inflammatory myositis, or antiphospholipid

Table 32-1 Interpretation of Synovial Fluid Cell Count

Leukocyte Count (WBCs/mm3)

Interpretation

<200

Normal synovial fluid

<2000

Noninflammatory fluid

>2000

Inflammatory fluid

2000-20,000

Mild inflammation (e.g., SLE)

20,000-50,000

Moderate inflammation (e.g., RA, reactive arthritis)

50,000

Severe inflammation (e.g., sepsis, gout)

>100,000

Sepsis, until proved otherwise

From Towheed TE, Hochberg MC. Acute monoarthritis: a practical approach to assessment and treatment. Am Fam Physician 1996;54:2239.

WBCs, White blood cells; SLE, systemic lupus erythematosus; RA, rheumatoid arthritis.

antibody syndrome, respectively (American College of Rheumatology, 1996).

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