Understanding Your Child’s Lab Results: What “High” and “Low” Really Mean

A flag is a comparison, not a diagnosis

When a result is marked “High” or “Low,” it usually means the number falls outside a “reference range”—the range of results seen in most healthy people. It does not tell you why the value is where it is, whether it matters, or what to do about it.

Many children with a flagged result are healthy. A result just outside the range may not need treatment, but your child's clinician should decide what it means.

The reverse is also true: a “normal” result does not rule out every problem. Some conditions can be present even when a particular test looks normal, which is why clinicians rarely rely on a single number in isolation.

Reference ranges leave room for normal differences between people. A small flag does not automatically mean your child is sick.

Why children need their own reference ranges

Children are not small adults. The expected amount of many substances in the blood changes as a child grows, sometimes month to month in infancy and again during puberty.

A few examples:

• Newborns normally have higher hemoglobin—the oxygen-carrying protein in red blood cells—than older infants or toddlers. Levels change quickly during early infancy.
• Alkaline phosphatase, an enzyme found in bone and liver, is often higher during growth. A value above an adult's range may be normal for a growing child.
• After puberty, some values differ between boys and girls. For example, hemoglobin tends to be higher in boys.

Because of this, results need to be interpreted using ranges appropriate for your child's age and, for some tests, sex. Adult ranges may give the wrong impression.

Why ranges differ from one lab to another

If your child has tests done at two different labs, you may notice that the ranges printed on the reports are not identical. Different labs use different machines and methods, so their reference ranges can differ.

The practical takeaway: use the range printed on that same report, and let your child's clinician check that it is appropriate for their age. Be cautious about comparing an exact number from one lab directly against another's—or against a range found online.

What common tests actually measure

• Complete blood count (CBC): Measures red blood cells, white blood cells, and platelets. It can help identify anemia—a lower-than-expected hemoglobin level—and changes in cells that fight infection or help blood clot. Results are interpreted together with your child's history and exam, not one value at a time.
• Iron tests, including ferritin: Help assess whether your child has enough iron. Ferritin helps estimate stored iron, but infection or inflammation can raise it and hide an iron shortage. Your clinician may consider other results or repeat the test after an illness.
• Electrolytes, such as sodium and potassium: Give information about the body's fluid and mineral balance. Sometimes the way a sample was collected affects the result. For example, blood cells breaking during collection or handling can make potassium appear higher than it really is. Do not assume that a high result is a sample problem—your clinician decides whether it needs urgent attention or repeat testing.
• Kidney tests, such as creatinine and BUN: Help assess how well the kidneys are filtering. Creatinine varies with age and muscle mass. Hydration can also affect these results.
• Liver tests, such as ALT, AST, and alkaline phosphatase: Can give clues about the liver, but some also reflect other parts of the body, including muscles and growing bones. Some changes are temporary. Your clinician considers which tests are affected, how much they have changed, and how your child is doing.
• Urine tests, or urinalysis: Check for signs of infection, blood, protein, and how concentrated the urine is. How the sample was collected can affect the results, so your clinician may request a fresh sample when a finding is unclear.

Why the whole picture matters more than the number

Your child's clinician reads a lab result alongside several things a portal cannot show:

• Symptoms and physical exam: The same value can mean different things in a thriving, playful child versus one who is unwell.
• The reason the test was ordered: A test done to investigate a specific symptom is considered differently from one done for screening.
• Trends over time: A single value is a snapshot. How a number is changing across several tests can help your clinician understand what is happening.

This is also why a value flagged “abnormal” may need no treatment, while a value within the “normal” range may still prompt further steps if it does not fit the clinical picture.

Why your clinician may want to repeat a test

Being asked to repeat a test does not necessarily mean something was missed or that the situation is worsening.

Repeating a test can help:

• Confirm whether a finding is real or was affected by the way the sample was collected.
• Show whether a result is improving, staying the same, or getting worse.
• Recheck a value after a temporary factor—such as an illness or dehydration—has passed.

Your clinician will tell you when to repeat the test. Some results need a quick recheck; others can wait.

What to do while you wait for an explanation

• Follow any urgent instructions your clinician's office has already given, including specific advice about medicines, fluids, or returning for care.
• Know when to expect a response. Ask how and when results will be explained. If you have not heard within that timeframe, contact the office that ordered the tests. Do not assume that no news means every result is normal.
• Try not to diagnose from the portal. Red flags alone or internet searches of individual values can cause needless worry.
• Write down your questions and any symptoms you have noticed, so you are ready to discuss them at the visit or call.
• Do not delay care for concerning symptoms while waiting on results. Seek medical care right away for trouble breathing, a stiff neck, a seizure, severe or worsening pain, persistent vomiting, signs of dehydration, a rash that does not fade when pressed, or a child who looks very unwell.

For a possible life-threatening emergency, call 911. Our Pediatrician, Urgent Care, or ER? guide explains how to choose the right level of care.

The bottom line for parents

A flagged result does not automatically mean something is wrong. Your child's clinician interprets it alongside their age, growth, symptoms, examination, and previous results.

Ask for an explanation, follow the recommended plan, and seek help promptly for critical results or a child who is unwell.