Laboratory studies provide a limited but useful role in orthopedics. The studies can be combined to reduce the number of needle aspirations.
Order a complete blood count and erythrocyte sedimentation rate and C-reactive protein as part of a screening evaluation to assess the general health of the patient, or when infection, neoplasm, or hematologic conditions are suspected.
The ESR is valuable in differentiating infections from inflammation and traumatic conditions. The CRP elevates more rapidly and returns to normal sooner than the ESR. The upper range of value for the ESR is 20 mm/hr. Inflammatory conditions such as toxic synovitis may raise the ESR to the 20–30 mm/hr range, but ESRs above 30 mm/hr are usually due to infection, neoplasm, or significant trauma. Except in the neonate, the CRP and ESR are usually always elevated by infections such as septic arthritis and septic osteomyelitis. In contrast, a leukocytosis is a less consistent finding.
Serum studies of calcium metabolism are occasionally useful when the possibility of conditions such as rickets is suspected. The normal range of these values is age dependent.
Screen for muscular dystrophy by ordering a creatinine phosphokinase (CPK) determination. Order the test if the young child appears weak, shows a clumsy gait, and has tight heel cords.
Chromosome studies are indicated for evaluating syndromes with features suggestive of a genetic disorder. These features include multiple system congenital malformations; mental retardation of unknown cause; abnormal hands, feet, and ears; and skin creases.
Bone Mineral Content
Mineral content of bones can be quantitated using several techniques. Cortical measurements can be made by radiography. The second metacarpal is a common standard. Single and dual photon absorptiometry are other alternatives. These studies are indicated for metabolic diseases, idiopathic osteopenia, and similar disorders.
Joint fluid should be visually examined and also sent to the lab for cell counts, chemistry, culturing, and staining. The joint sugar is usually about 90% serum level and is reduced in infection. In about one-third of cases of septic arthritis, cultures are negative.