Laboratory Studies

CHAPTER 6

Laboratory Studies

• Laboratory studies can help identify infection (eg, osteomyelitis, septic arthritis) or systemic disease.

• A complete blood cell count can be helpful in the diagnosis of many types of systemic illness. An elevated white blood cell (WBC) count with an increase in immature cells (bands) is highly suggestive of bacterial infection. A decrease in multiple cell lines raises concern for hematologic malignancy. Elevated platelets suggest systemic inflammation.

• Lactate dehydrogenase may be elevated in patients with malignancies.

• Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are nonspecific inflammatory markers.

— A slightly elevated ESR in the range of 20 to 30 mm/h suggests an inflammatory condition such as juvenile arthritis or may represent viral infection or a normal variant.

— In the presence of infection, neoplasm, or significant trauma, ESR is typically above 30 mm/h.

— Compared with ESR, CRP has higher sensitivity and specificity for infection.

— CRP level tends to rise early and decline early; conversely, ESR may be normal early in the course of infection and remain elevated for a prolonged period. As a result, CRP level is a better marker earlier in the course of illness or for monitoring early response to treatment, whereas ESR is a better marker for detecting persistent inflammation and for determining duration of treatment.

• Blood cultures are crucial in evaluating for infection in children, as most infections are due to hematogenous spread.

— Because of the significant rate of false-negative cultures, multiple samples should be obtained when feasible.

• Cultures of potentially infected sites (eg, bone, joint, soft tissue) should be obtained when identified.

• Synovial fluid analysis is an important tool in the diagnosis of monoarticular arthritis.

— Greater than 2,000 WBCs per milliliter of synovial fluid suggests an inflammatory process.

— Greater than 50,000 WBCs per milliliter is highly suggestive of septic arthritis, while greater than 100,000 WBCs per milliliter is virtually diagnostic of septic arthritis (Table 6-1).

• Creatine kinase (also known as creatine phosphokinase) is used to detect muscle breakdown as can be seen in children with muscular dystrophy.

Table 6-1. Synovial Fluid Characteristics

• Thyroid studies identify hypothyroidism associated with delayed skeletal maturation, atypical slipped capital femoral epiphysis, or Perthes disease.

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