Lab Tests & Children

Lab tests are procedures in which a sample of body tissue or fluids is obtained and analyzed to check for signs of illness. They are often performed when a physical examination or patient’s list of symptoms indicate the potential presence of disease.

In other cases, lab tests are performed as part of routine health maintenance, such as newborn screening tests performed after a baby is born. These tests can help detect conditions that may lead to physical problems, mental retardation and death. In many cases, early diagnosis of these disorders can lead to treatments to manage the condition and minimize symptoms.

A baby’s first lab tests occur shortly after birth. Most newborn screening tests are performed by analyzing a drop of the baby’s blood obtained by pricking the baby’s heel. This sample is then analyzed for illness through the use of several different techniques.

Children older than 1 year of age occasionally undergo screening tests for conditions such as tuberculosis or high blood levels of lead (plumbism). However, many older children generally do not undergo lab tests unless a physical examination or history of symptoms indicates the potential presence of an illness or disorder. When such illness is suspected, a lab test can help confirm a diagnosis or indicate a need for further testing. There are scores of lab tests available to a physician. Some tests may be inappropriate for the condition that is suspected, whereas others are not sensitive enough to reveal a problem.

Lab tests that are commonly performed to diagnose illness in children include:

Lab tests are routinely performed to screen newborns for signs of developmental or genetic disorders. Identifying such conditions early – particularly before symptoms develop – can lead to early treatment of the disorder, which is often more effective than treating a problem in a later stage of development. The tests can be particularly useful when rare metabolic disorders are identified. These conditions may involve buildup or inability to process certain chemicals, which may lead to mental retardation or coma and death. Strict diets that avoid certain foods can minimize or eliminate these effects. For this reason, these tests are often lifesaving tools.

The lab tests used to screen newborns do not actually diagnose a condition, but merely indicate that further testing is necessary. In fact, most children who have a positive result after these screenings turn out not to have the disease or disorder (false-positive result).

The best screening tests have low false-negative rates, but may have high false-positive rates. This is because it is more important that a test falsely indicates presence of a disease that is not there than to miss low levels of a disease that is present.

Each state has its own regulations for which screening tests are required of newborns, and the number of tests typically ranges between 3 and 10. Some states require as many as 40 tests. All 50 states screen for congenital hypothyroidism, which involves poor growth and brain development as a result of having low levels of thyroid hormone. When detected early, this condition can be treated with thyroid hormone supplements.

Other conditions that may be screened for – roughly in order of the number of states that require testing for the particular disorder – include:

Although each state has its own list of a handful of required lab screenings for newborns, many other lab tests also are available. The March of Dimes suggests more than two dozen tests that parents are urged to consider. However, in many cases medical insurance does not cover the cost of these extra tests.

Parents may want to consider testing for a disorder if there is a family history of the condition, a newborn’s sibling has the condition, an infant in the family has died because of the disorder or there is some other factor that puts the newborn at high risk for the disease. In addition, if a pregnant woman has been exposed to certain infections, she and her baby may be tested for various infections.

As children grow older, they may undergo other screening tests. For example, children may undergo testing for tuberculosis, which is evaluated with a skin prick test called the PPD (purified protein derivative). Parents are asked to watch the pricked area for 48 to 72 hours to see if any redness, hardness or blistering appears. Children also may have their iron and lead levels tested through taking a sample of blood.

Some physicians may recommend that some lab tests be performed at regular intervals, such as a urine test to assess kidney function. In many cases, however, lab tests are performed on children only in circumstances where a physical examination or medical history indicates the possible presence of a disease. For example, a physician may recommend that a child or adolescent who is obese and has a family history of diabetes mellitus undergo a blood test to check the level of glucose (blood sugar).

No preparation is required for newborn testing, which usually takes place in the hospital between 24 hours and seven days after a child is born. Parents should have their children follow any preparatory directions provided by the physician or other healthcare professional.

Lab tests may be performed differently depending on the nature of the tests. During a newborn test, a nurse or other healthcare professional will prick the baby’s heel to obtain a small amount of blood. This will be sent to a laboratory for analysis.

The technique for drawing blood depends on how much blood is needed. If the test requires only a drop or two of blood, then a simple prick of the finger or earlobe can provide enough blood for testing. A sterile, sharp lancet is used to prick the skin. The technician will then gently squeeze the puncture area to produce drops of blood that are collected in tiny glass tubes. Light pressure and sterile gauze are then applied to the puncture site to stop the bleeding. A bandage may be applied.

If a substantial amount of blood is needed, it is usually drawn from a vein in a process called venipuncture. During venipuncture, a needle is inserted into a vein – usually at the inside of the elbow or on the back of the hand. The area around the puncture site is cleaned with rubbing alcohol and a wide elastic band or piece of latex tubing may be placed around the upper arm to slightly increase pressure in the vein. One end of a sterile double-ended needle that has been attached to an open-ended syringe (which contains an empty test tube) is inserted into the vein. Because the test tube contains a partial vacuum, blood flows directly from the vein through the double-ended needle and into the test tube.

After the necessary amount of blood is drawn, the needle is withdrawn and a small cotton ball or pad is applied with light pressure over the puncture site. After several minutes, the cotton will be discarded or replaced, and a small bandage will be placed on the puncture wound. The entire process takes less than 10 minutes.

Collecting a urine sample usually takes just minutes to complete. Performed in a physician’s office, the urine test will require a 1- to 2-ounce (30- to 60-milliliter) sample to be collected in a cup. The urine sample should not be brought from home because the urine needs to be as fresh as possible. The child will provide the sample by urinating into a receptacle used for this purpose.

During throat cultures, children are asked to open their mouths wide while a long cotton swab is inserted and the back of the throat is wiped.

Samples for stool tests are often collected in the home. Parents will likely need to help their young children with this process, which is explained in detail by a physician.

During a spinal tap, infants and small children lie on their sides curled up in a position that resembles the letter “C,” with their knees under their chin. Teens lie on their sides with their knees drawn up to their chest and chin resting on their chest. A local anesthetic is used to numb the lower back, and a small, hollow needle is inserted near the waist to draw out a sample of cerebrospinal fluid (CSF).

The sample obtained during these procedures is analyzed by a professional in a laboratory.

Tests of newborns that are performed less than 24 hours after the child’s birth may not be as sensitive as those performed later. If the baby is tested within 24 hours of birth, lab tests – particularly for phenylketonuria (PKU) – should be repeated within two weeks.

Other lab tests may require modifications to the child’s diet or medication regimen. Consuming certain foods or beverages or taking certain drugs can skew the results of some tests. For example, consuming peroxidase compounds such radishes, turnips, cantaloupe, bean sprouts, cauliflower, broccoli or grapes prior to a stool test can result in a color change that resembles blood. This could lead to a false positive result.

Following a lab test, a sample is sent to an expert who analyzes the sample. Lab test results are evaluated in relation to the “normal range” for that test. The range of values considered to be normal is the range of test results from the blood of normal, active, healthy children. Children who have a disease or health problem often have tests results that are higher or lower than normal – or “outside of the normal range.”

When a physician sees that a lab test is outside of the normal range, the test may be repeated to verify results or additional tests may be ordered to determine the underlying cause of the abnormality.

Normal ranges for some tests may vary slightly from lab to lab. For example, blood test ranges will differ between labs that use machines to perform blood tests and those in which blood tests are performed by hand.

Frequently, results of a child’s lab test are compared to another “known” blood sample taken from a healthy individual that is run at the same time and is designated as the “normal control.” When the “normal control” sample falls within the normal range, or reaches a specific “known” measurement, the laboratory results confirm that the lab test has been carried out accurately.

If a child’s lab test is not designated as “normal,” that does not necessarily mean that the child has the condition for which he or she has been tested. In some cases, more specialized testing (such as a biopsy) or other testing techniques (such as imaging tests) may be necessary to confirm the findings of the initial lab test.

In many cases, a single lab test result may be enough to establish a diagnosis in a child who is suffering symptoms related to an illness. In other cases, children may undergo periodic testing to monitor the progression of a medical condition, or to check the effectiveness of a treatment plan over a period of time.

Most healthy children do not undergo lab tests on a regular basis. Occasionally, a lab test may be used to screen a child for certain disorders, such as high levels of lead in the blood or to check for the presence of tuberculosis.

There are few risks associated with lab tests. Newborns are likely to feel pain and to cry when their heel is pricked. There may be a small amount of bruising following the test.

Children who have blood drawn also may experience minor bruising at the needle insertion site. This can be minimized by keeping direct pressure on the spot for several minutes after the needle has been removed.

In rare situations, the vein used to obtain the sample may become inflamed (phlebitis) or obstructed (thrombosis). Applying warm compresses to the spot several times daily can help reduce the swelling.

Children with bleeding disorders and those who are taking anticoagulants (blood thinners) may have problems with continued bleeding. Children who experience these side effects should be taken to their physician.

There is a small risk of infection with a spinal tap. Other rare side effects that may occur include allergic reaction or damage to the spinal cord.

Treatments or additional tests that follow lab tests will vary depending on the disorder that is detected or suggested. For example, many symptoms associated with disorders screened for in newborn tests can be prevented with changes in diet and taking certain supplements.

Lab tests that reveal the presence of disease can lead to many different treatments, depending on the nature of the disease. These range from medications such as antibiotics to surgery. In some cases, further testing may be necessary, such as imaging tests (e.g., x-ray, ultrasound, MRI, CAT scan) and a biopsy, to confirm a diagnosis.