Interpreting Rheumatology Laboratory Tests

Interpreting pediatric rheumatology laboratory tests can seem daunting. This In Brief describes key rheumatology laboratory tests related to the evaluation and management of childhood rheumatologic diseases. Most pediatric rheumatology diseases are clinical diagnoses in which laboratory tests help confirm physicians’ clinical reasoning. Hence, pediatricians need to be familiar with the clinical manifestations of the different entities to inform the ordering of appropriate tests. However, general laboratory tests can be completed to evaluate for mimicries such as infection and malignancy. When in doubt, a referral to a pediatric rheumatologist is advised.Nonspecific inflammatory markers, including C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR), can be useful components of diagnosing and managing pediatric rheumatologic diseases. CRP is a protein produced by the liver in response to inflammation, which can be elevated in infectious, malignant, and rheumatologic processes. Among rheumatologic diseases, the CRP level is often highly elevated in systemic juvenile idiopathic arthritis (JIA), multisystem inflammatory syndrome in children, Kawasaki disease, and periodic fever syndrome flares. Due to a relatively short half-life, it is useful for trending levels of inflammation. Specific estimates vary, but in general, CRP rises within a few hours of the onset of inflammation (half-life of 19 hours) and declines rapidly once inflammation subsides. The CRP level can often be normal in systemic lupus erythematosus (SLE), nonsystemic JIA, and juvenile dermatomyositis. In children with these diseases, an elevated CRP level likely indicates infection rather than flare, particularly if the child is taking immunosuppressant medications. Because CRP is synthesized in the liver, this test is not as useful in patients with liver disease.ESR is a nonspecific marker of inflammation that measures how quickly the red blood cell level declines. Normally, negatively charged erythrocytes repel each other. Positively charged inflammatory proteins, primarily fibrinogen, attract the erythrocytes, leading to stacks of cells (rouleaux) that fall faster than individual erythrocytes. The half-life of ESR is longer than that of CRP (days rather than hours); thus, the ESR reflects longer-term patterns of inflammation, such as chronic inflammatory conditions like SLE. The ESR can be falsely elevated after intravenous immunoglobulin administration due to immunoglobulins, and in anemia because erythrocytes fall more quickly with fewer present. The ESR can be falsely lowered in sickle cell disease and spherocytosis because abnormal erythrocyte shapes prevent rouleaux formation. CRP level and ESR can be used to monitor responses to treatment of rheumatologic diseases.When a patient with JIA has a highly elevated ESR or CRP measurement, accompanied by extremely elevated levels of ferritin, fibrinogen, and/or D-dimer, the clinician should suspect systemic JIA and its life-threatening complication, macrophage activation syndrome. A rising CRP level in the context of a declining ESR can be useful for monitoring for macrophage activation syndrome because when fibrinogen (the main component of ESR) is quickly consumed, the ESR drops and the CRP level stays elevated. Therefore, a high CRP level and a normal ESR can indicate extremely high levels of inflammation and, thereby, a worse prognosis.Muscle enzymes can be useful in the diagnosis and monitoring of pediatric rheumatologic diseases. Muscles are rich in the enzyme creatine kinase; thus, muscle damage releases creatine kinase into the bloodstream. Aldolase and lactate dehydrogenase are enzymes involved in metabolic pathways that increase when muscles are stressed. Transaminases (aspartate aminotransferase and alanine aminotransferase), although generally used to assess liver inflammation or injury, are also present in muscles and salivary glands. Children with juvenile dermatomyositis often have elevations of at least 1 of these muscle enzymes. However, if diagnosis is delayed and/or the patient presents with significant muscle wasting, muscle enzyme levels can be falsely lowered because the muscle is too atrophied to release significant enzyme quantities. Aldolase and lactate dehydrogenase levels can be falsely elevated in hemolyzed samples.Antinuclear antibodies (ANAs) target components in the cell’s nucleus. The ANA test includes several different ANAs, including anti–double-stranded DNA, anti-Smith, anti-Ro/anti-SSA (anti–Sjögren syndrome–related antigen A), anti-SSB (anti–Sjögren syndrome–related antigen B), anti-RNP (U1 ribonucleoprotein), anticentromere, and antihistone antibodies. The most accurate and dependable ANA test in the pediatric population is the indirect fluorescence antibody (“IgG by IFA”) test, as the enzyme-linked immunosorbent assay (ELISA) test has high false-positives and false-negatives in the pediatric population. When an ANA test is positive through IgG by IFA, it is important to look at the titer, which describes the quantity of antibody present. There are different estimates regarding the clinical significance of titers. Based on clinical experience, our preference is to consider a titer of 1:80 as a clinically low positive ANA and is rarely associated with autoimmune disease. When the titer increases to 1:640, it is much more likely to be associated with a pediatric autoimmune disease, and a titer of 1:1,250 is highly concerning for autoimmune disease. The pattern of ANA refers to immunofluorescence uptake and varies based on the particular antibody. For example, the double-stranded DNA creates a homogenous stain because DNA is found throughout the nucleus, whereas anticentromere antibodies stain only in the location of the centromeres.Although ANA is commonly thought of as a screening laboratory test for general autoimmunity, ANA testing should not be ordered as a screening test for children without clinical manifestations of autoimmunity. Approximately 15% to 20% of the healthy childhood population have a positive, but clinically insignificant, low-titer ANA. Moreover, children with autoimmune diseases do not have a positive ANA. When children have clinical presentations concerning for SLE, mixed connective tissue disease, or Sjogren syndrome, a positive ANA can aid in diagnosis. Among children with JIA, the ANA helps to assess the risk of anterior uveitis, which is more likely with a positive ANA and is often an asymptomatic manifestation of the disease. Because untreated anterior uveitis can lead to vision loss, ANA-positive patients with JIA should have close ophthalmology follow-up.In children with SLE, autoantibodies can lead to activation of the complement system, which is a group of proteins that destroys bacteria and viruses through a cascade. Low C3 and C4 complement levels can indicate disease flares, particularly in lupus nephritis, and thus help to monitor disease activity. Low complement levels are not specific to SLE, but they offer 1 clue in the diagnostic puzzle.Rheumatoid factor (RF) is an antibody that binds to the common portion of IgG antibodies, creating an inflammatory cascade that targets the synovial lining of joints. Unlike rheumatoid arthritis in adults, JIA is a purely clinical diagnosis, which means that laboratory tests, including RF, are not part of the diagnostic criteria. Many children with JIA have a negative RF. In addition, RF can be positive in a variety of nonarthritic autoimmune, infectious, and malignant processes. However, in children with JIA, RF can be useful for classifying the type of JIA and informing prognosis and management. A positive RF suggests more erosive and aggressive disease, as does a positive anti–cyclic citrullinated peptide antibody, which targets citrullinated proteins present in inflamed synovial fluid.Human leukocyte antigen (HLA-B27) is a genetic marker that is common in the general population, particularly among American Indian and non-Hispanic white populations. It is not useful as a screening test. For example, a child with joint pain without a diagnosis of JIA should not have an HLA-B27 test as part of the diagnostic evaluation. Among patients with JIA, HLA-B27 helps classify the arthritis subtype (a positive HLA-B27 test increases the likelihood of enthesitis-related arthritis).Antineutrophil cytoplasmic antibodies (ANCAs) are associated with ANCA vasculitis, but their role in pathogenesis is unclear. There are 2 ways to test ANCA antibodies: IFA and ELISA. In the IFA test, the antibodies result as cytoplasmic ANCA, perinuclear ANCA, and atypical ANCA. In the ELISA test, the antibodies result as anti–proteinase 3 and myeloperoxidase. Positive cytoplasmic ANCA and anti–proteinase 3 tests are specific for a type of vasculitis called granulomatosis with polyangiitis. Positive perinuclear ANCA, myeloperoxidase, and atypical ANCA test results are much less specific; they can be present in vasculitis but also in other autoimmune diseases, such as inflammatory bowel disease.Interpreting pediatric rheumatology laboratory tests can be complex. The ability to recognize clinical manifestations of childhood rheumatic diseases is essential for the selection of appropriate tests for the evaluation of childhood rheumatologic diseases.This In Brief makes several critically important points: that clinicians need to guide their diagnostic clinical reasoning by being aware of the manifestations of specific rheumatologic diseases, and avoid indiscriminate testing with a batch of “rheumatology tests” but instead use discretion in selecting those tests that will help in confirming the diagnosis. I have found as both a clinician and an educator that with the easy accessibility of testing, clinicians may be prompted to order a large number of tests without the backing of diagnostic reasoning. Yet, it is imperative in providing high-quality care to order tests when they are indicated by clinical reasoning because false-positive results can lead to added expense in addition to enhanced patient and family anxiety and worry. I have found that partnering with a rheumatology specialist has been essential when I have been uncertain as to the ordering or interpretation of tests to provide the best care to patients.–Janet Serwint, MDAssociate Editor, In Brief