Because there are numerous laboratory tests available, the physician must use the patient's history and physical to accurately determine which laboratory tests to perform. Commonly used tests include a complete blood count (CBC), acute-phase proteins [erythrocyte sedimentation rate (ESR), c-reactive protein (CRP)], blood chemistry, rheumatologic, and infectious disease studies.
The CBC helps provide an estimate of a person's general health. Based on the hematocrit level, an indication of that person's medical and nutritional health can be inferred. The shape of red blood cells allows for the determination of pain-inducing diseases such as sickle cell anemia. White blood cells when elevated can point to infections or underlying hematologic malignancies. Similar to white blood cells, platelet levels help elucidate underlying myeloproliferative disorders. Platelet levels also influence whether the patient is a candidate for invasive therapeutic procedures.
Acute-phase proteins such as the ESR and CRP provide a general indication of inflammatory issues within the patient. Abnormal values of these two tests are often seen with infection, trauma, surgery, burns, cancer, inflammatory conditions, and psychological stress and also help corroborate findings of thrombocytosis, leukocytosis, and anemia.
Coagulation parameters are a useful laboratory test as they determine the potential application of invasive therapeutic pain treatments as well as provide an assessment of the patient's liver function. Deficiencies in the clotting studies should make the practitioner wary of unrecognized bleeding into limited spaces (retroperitoneal, joints) as a possible cause for a patient's pain.
Blood chemistry values include sodium, urea, creatinine, and glucose. While hypona-tremia itself may cause generalized symptoms of pain, the physician should determine whether a patient's hyponatremia is indicative of an abnormal hormonal process such as syndrome of inappropriate antidiuretic hormone (SIADH), which occurs with certain types of cancers. Monitoring glucose and hemoglobin A1c levels may uncover diabetes, a common cause of painful neuropathies. Abnormalities in the urea and creatinine levels can indicate issues of renal insufficiency, which may alter the pharmacologic and invasive treatment options. For example, worsening renal function may worsen side effects from morphine because of decreased excretion of morphine metabolites.
Diseases such as systemic lupus erythematosus and rheumatoid arthritis are associated with diffuse body pain and are characterized by inflammation of the joints, muscle, or skin. Screening for rheumatologic disorders includes testing for autoantibodies, antinuclear antibodies (ANA), rheumatoid factor, antineutrophil cytoplasmic antibody, anti-Ro, anti-Sm, and anticentromere. Diffuse pain and joint pain warrant consideration of a rheumatologic evaluation.
Because certain infectious diseases produce generalized pain symptoms, screening for diseases like HIV, syphilis, and lyme disease should be performed when indicated. HIV commonly causes abdominal pain, neuropathies, oral cavity pain, headaches, and reactive arthritis. Spirochetal diseases like syphilis and lyme disease can range in severity ofsymptoms including headaches, irritability, neck stiffness, or gummata. The Venereal Disease Research Laboratory (VDRL) and rapid plasma regain (RPR) tests are the initial tests used to screen for syphilis. Lyme disease, like syphilis, can range in pain symptoms from cranial neuritis, radicular pain, and weakness, to symptoms of Bell's palsy. Screening for lyme disease typically requires an enzyme-linked immunosorbent assay (ELISA) test.
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