Test	Condition	Expected finding	Comments
ANA	SLE	Positive	Present in 94 percent of children with SLE11; a negative ANA test virtually rules out SLE.12
			The test’s predictive value is low in most nonspecialty settings; most positive results do not indicate SLE.13
			Ten to 40 percent of healthy children can have a positive test.12,14
			Test can be positive in patients with other medical conditions (e.g., infection, malignancy, other autoimmune conditions).13,15
			A titer between 1:160 and 1:320 offers the best combination of high sensitivity and high specificity.12
			A positive test by itself is not diagnostic for SLE; three additional criteria must be present.16
			Test is of no diagnostic utility in ruling in or ruling out juvenile rheumatoid arthritis.17
ASO	Acute rheumatic fever	Increased ASO titers	Indicates true infection rather than carriage.
			Elevated ASO titers are found in up to 80 percent of patients with acute rheumatic fever.18
			Sensitivity can be further increased by testing for additional antibodies.18
Blood culture	Infection	Positive	Test is positive in 30 to 60 percent of patients with osteomyelitis19 and in 40 to 50 percent of patients with septic arthritis.20
Bone culture	Osteomyelitis	Positive	Test is positive in 48 to 85 percent of patients with osteomyelitis.19
			Staphylococcus aureus is the most common pathogen isolated.19
CBC	Infection	Increased WBCs and platelets	WBC count is neither sensitive nor specific for infection, inflammation, or malignancy.
	Inflammation	Increased WBCs and platelets	Blast cells, lymphocytosis, and neutropenia may be seen in patients with leukemia.21
	Malignancy	Cytopenia22	Cytopenia may occur in patients with SLE.11
Coagulation profile	Known hemophilia or hemorrhagic effusion	Increased activated partial thromboplastin time	—
CRP	Infection	Increased CRP levels	Test is neither sensitive nor specific for infection, inflammation, or malignancy.
	Inflammation	Increased CRP levels	The negative probability of septic arthritis is 87 percent when CRP level is > 1 mg per dL (10 mg per L).23
	Malignancy	Increased CRP levels	In patients with osteomyelitis and septic arthritis, CRP levels should rapidly normalize after initiation of therapy. A persistently elevated CRP level after the initiation of antibiotics indicates a poor response to therapy.24,25
ESR	Infection	Increased ESR	Test is neither sensitive nor specific for infection, inflammation, or malignancy.
	Inflammation	Increased ESR	The negative probability of septic arthritis is 85 percent when ESR is > 25 mm per hour.23
	Malignancy	Increased ESR	A low or normal platelet count in the presence of an elevated ESR suggests malignancy.22
Lyme titer	Lyme disease	Positive	All children who live in or have recently traveled to an area endemic for Lyme disease should be tested.26
Synovial fluid analysis	Septic arthritis	Turbid synovial fluid; WBC count > 50,000 to 100,000 per mm3; PMNs > 75 percent27	—
	Transient synovitis	Clear yellow synovial fluid; WBC count 5,000 to 15,000 per mm3; PMNs < 25 percent27
Synovial fluid culture	Septic arthritis	Positive	Test is positive in 50 to 80 percent of patients with septic arthritis.20
	Transient synovitis	Negative	S. aureus is the most common pathogen isolated in patients with septic arthritis.27
Throat culture	Acute rheumatic fever	Group A hemolytic streptococci	Positive in only 10 to 33 percent of patients with acute rheumatic fever.18
Urethral, cervical, pharyngeal, and rectal cultures	Gonococcal arthritis	Neisseria gonorrhoeae	—
Urethral and stool cultures	Reactive arthritis	Chlamydia in urethral cultures28; Salmonella, Shigella, Yersinia, and Campylobacter in stool cultures28	—