Understanding the Different Lab Tests for Juvenile Arthritis
What do the lab tests your child’s doctor orders mean?
By Mary Anne Dunkin
Like most parents, you’ve guided and encouraged your child in her accomplishments. You held her hand as she took her first steps, and you beamed with pride when she recited her ABCs. But when your child stepped into the world of juvenile arthritis and its management, you may have felt as if you were the one who needed hand-holding. It can seem there is a whole new alphabet to learn – when your child’s doctor tells you he is ordering a CBC, an ESR or a UA, that your child has a positive ANA or negative RF, or that she’ll need regular LFTs.
Whether your child is just beginning the diagnostic process or is well into the management of arthritis, laboratory tests – which doctors often refer to by names that sound like secret code – are a part of life. Tests on even a small sample of blood or other bodily fluid can give the doctor an important glimpse into what’s going on throughout your child’s body, says Ilona Szer, MD, director of pediatric rheumatology at the Children’s Hospital of San Diego. Your doctor can use lab tests to rule out other conditions, determine the extent of disease or monitor the activity of the disease or the effects – both good and bad – of the drugs used to treat it.
To help you crack the code and make sense of the tests your child’s doctor orders, Dr. Szer identifies some of the most common tests prescribed by pediatric rheumatologists and provide explanations of the tests, their limitations, and why doctors use them.
CBC (Complete Blood Count)
What it is: A measure of three types of cells that are present in the blood – red cells (RBCs), which carry oxygen; white blood cells (WBCs), which fight infection; and platelets, which make the blood clot.
Why doctors order it: Cell counts can provide clues to your child’s condition. For example, white cell counts are very high in systemic juvenile idiopathic arthritis (JIA) and low in lupus; platelets may be very low in lupus and very high in systemic JIA; anemia (low red blood cells) is almost always present in a child with systemic JIA. Some of the medications used to treat arthritis may cause gastric bleeding, and the RBC count might fall as a result.
CRP (C-Reactive Protein)
What it is: A test that measures the concentration of a protein called c-reactive protein in the blood. The higher the concentration, the more severe the inflammation.
Why doctors order it: To figure out the extent of inflammation. Because the test is sensitive and responds rapidly to changes in inflammation, your child’s doctor may use it to monitor her disease activity and response to treatment.
Test limitations: The CRP goes up any time there is inflammation, including when your child has a cold.
ESR (Erythrocyte Sedimentation Rate – also called “sed rate”)
What it is: A test that measures how fast RBCs, or erythrocytes fall and settle, like sediment, at the bottom of a test tube. The presence of inflammatory factors affect the rate at which the cells fall. The more inflammation in the body, the higher the ESR.
Why doctors order it: Because a high ESR, like a positive CRP, indicates the presence of inflammation. In a child already diagnosed with arthritis, a doctor may use the test to monitor disease activity or response to treatment. A lower sed rate would indicate inflammation is being controlled.
Test limitations: The ESR is less sensitive than the CRP and doesn’t respond as quickly to changes in inflammation, so many doctors prefer the CRP.
What it is: A test in which urine is examined under a microscope.
Why doctors order it: Because some medications are metabolized or excreted by the kidneys, the doctor will use a urinalysis to be sure the kidneys aren't overburdened. Doctors also use the test to look for and monitor lupus-related kidney disease.
CPK (Creatinine Phosphokinase)
What it is: A test used to detect the presence of one of several enzymes released from muscle cells when they are damaged by inflammation.
Why doctors order it: An elevated CPK raises the suspicion for juvenile dermatomyositis (JDM), because muscle inflammation and subsequent damage are the hallmarks of JDM. The test also measures a medication’s effectiveness in reducing the inflammation that caused the damage.
LFT (Liver Function Test)
What it is: A test that determines whether the liver is healthy and functioning normally. The most commonly ordered tests are liver transaminases, which measure damage to the liver.
Why doctors order it: Because many medications used to treat arthritis (methotrexate, in particular) are metabolized in the liver and may overburden the liver, your child’s doctor may order regular LFTs to determine if the medications are causing any damage.
HLA (Human Leukocyte Antigen) – B-27 typing
What it is: A blood test used to detect the presence of a B27 variant of a gene involved in immune regulation.
Why doctors order it: Detection of HLA-B27 tells the doctor that a child has a genetic marker associated with a spondyloarthropathy, such as ankylosing spondylitis (AS) or a related condition known as enthesitis related arthritis (ERA) or seronegative arthritis and enthesopathy (SEA) syndrome. These syndromes are frequently suspected in older children (boys more than girls) with hip and knee arthritis and back pain. Ninety-five to 98 percent of patients with AS have a positive HLA-B27, as do 50 percent of those with other spondyloarthropathies.
Test limitations: Eight percent of Caucasians have a positive HLA-B27, while far less than one percent actually develop arthritis.
RF (Rheumatoid Factor)
What it is: A test designed to detect and measure levels of a specific antibody directed against the blood component immunoglobulin G (IgG).
Why doctors order it: A positive RF is found in adults with rheumatoid arthritis. If the test is positive, it helps your child’s doctor determine both the form of JRA he or she has and how aggressively to treat it. “Children – primarily girls – with RF-positive JRA truly have rheumatoid arthritis and, therefore, are at greatest risk for joint erosions, rheumatoid nodules and poor functional outcome,” says Dr. Szer.
Test limitations: Fortunately, unlike adults, 90 percent of children who have JRA do not have a positive RF.
ANA (Antinuclear Antibody)
What it is: A test that measures the presence of an antibody directed against structures in the child’s cells’ nuclei.
Why doctors order it: A positive ANA is associated with many kinds of arthritis and with other rheumatic conditions such as lupus, scleroderma and mixed connective tissue disease. In young girls with pauciarticular JRA, a positive ANA may be associated with a higher risk of uveitis.
Test limitations: Many rheumatic conditions have a positive ANA, and up to 20 percent of all children have positive ANAs, so the presence of ANA alone doesn't necessarily indicate a problem.
Beyond Lab Tests
These are just a few of the more common tests your child’s doctor may order. There are also many other types of tests – including X-rays, bone scans and biopsies – that doctors may use to help them better understand your child’s disease. Yet every test has its limitations, and no test in itself can diagnose arthritis. The right tests, along with your doctor’s own observations and your participation in the process, can help your child get the right diagnosis and the safest and most effective treatment.