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. Common lab tests for newborns include:

• Acylcarnitine profile. Screens for organic acid and fatty acid disorders.
  
  
• Amino acid profile. Looks for amino acid deficiencies, which are found in maple syrup urine disease (MSUD) and phenylketonuria (PKU).
  
  
• Tandem mass spectrometry. Often known as MS/MS, this technology allows for a single test to screen for more than 20 inherited metabolic disorders.

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:

• Blood tests. Includes many different types of tests that can help detect signs of underlying illness. A complete blood count can reveal signs of infection, anemia (low levels of red blood cells) or cancer. Other blood tests can uncover signs of diabetes, liver or kidney problems, metabolic disorders, tuberculosis, hormonal disorders (such as thyroid, adrenal gland, or gonadal or pituitary gland problems) and tissue damage.
  
  
• Urine tests. Used to make sure that kidneys and other organs are functioning, or to diagnose conditions such as infections of the kidneys, bladder or urinary tract. 
  
  
• Throat culture. Tests for the presence of group Ab-hemolytic streptococci, which causes strep throat. Children with strep have sore throats and are at risk for developing more severe complications of streptococcal infections such as scarlet fever or rheumatic fever.
  
  
• Stool tests. Used to detect allergies or inflammation in the body, infections or bleeding in the gastrointestinal system, and digestive disorders (such as malabsorption of sugars, fats and nutrients). 
  
  
• Spinal tap. Procedure during which a long needle is inserted in the spinal canal in the lower back to collect cerebrospinal fluid (CSF) for laboratory analysis. This clear, watery fluid circulates around the brain and spinal cord. Spinal taps in children are most often performed to diagnose meningitis, an infection and inflammation of the membranes (meninges) covering the brain and spinal cord.