Reading lab results: what high, low and abnormal really mean
Where reference ranges come from, why one in twenty healthy people falls outside each of them, and how to read the common blood tests that show up in clinical cases.
A lab report looks precise: a number, a unit and a little flag that says H or L. But that flag is a statistical judgment, not a diagnosis. Knowing where it comes from makes lab results far easier to read, whether on a ward round or in a Syndrome case.
Where reference ranges come from
For most blood tests, the reference range (or reference interval) is built by measuring the test in a large group of healthy people and taking the middle 95% of their results. Anything below the 2.5th percentile is flagged low; anything above the 97.5th percentile is flagged high.
Two consequences follow directly from that definition:
- About 5% of healthy people fall outside the range on any single test. A mildly "abnormal" result in a well person is often just normal variation.
- The more tests you run, the more likely something gets flagged. If you ran 20 unrelated tests on a perfectly healthy person, the chance that at least one comes back outside its range is roughly 64% (because 0.95 multiplied by itself 20 times is about 0.36).
This is one reason clinicians try not to order large panels of tests without a question in mind.
Reference ranges also vary. They differ between laboratories and testing methods, and many depend on age, sex or pregnancy. Units differ between countries too: glucose, for example, is reported in mg/dL in the United States and mmol/L in much of the world. Always read a result against the range printed on its own report.
Context changes what a number means
A lab value is interpreted against the rest of the picture:
- Pretest probability. A slightly raised result in someone with typical symptoms means much more than the same result in someone with none.
- Trends. A creatinine that has doubled since last week matters more than one that has been stable for years. Some diagnoses depend on a change over time, such as a cardiac troponin that rises and then falls.
- Size of the deviation. A value just outside the range is common and often harmless; a value far outside it is rarely an accident.
- Critical values. Some results are dangerous in themselves and trigger an immediate phone call from the lab, such as a very high potassium or a very low glucose.
Quantitative and qualitative results
Most blood tests give a number that is compared with a range: high, low or normal. Other tests give a description: a urine dipstick shows protein, a culture grows bacteria, an ECG shows a rhythm, an X-ray shows a shadow. For those, the useful summary is normal or abnormal, and the details say what kind of abnormal.
Stat!'s Syndrome game follows the same convention. Blood tests come back high, low or normal; cultures and antibody tests come back positive or negative; descriptive tests such as a urinalysis or a blood smear come back normal or abnormal. A normal result is still information: a normal lipase, for instance, makes acute pancreatitis much less likely.
The common blood tests
The table below lists tests you will meet again and again, with typical adult reference ranges. Treat the numbers as approximate: every lab publishes its own.
| Test | Typical adult range | High suggests | Low suggests |
|---|---|---|---|
| Hemoglobin | About 13.5–17.5 g/dL (men), 12.0–15.5 g/dL (women) | Dehydration, polycythemia, living at altitude | Anemia: blood loss, iron or B12 deficiency, chronic disease |
| White cell count | About 4.0–11.0 × 10⁹/L | Infection, inflammation, steroids, leukemia | Viral infection, bone marrow problems, some medicines |
| Platelets | About 150–450 × 10⁹/L | Inflammation, iron deficiency, marrow disorders | Immune destruction, marrow failure, a large spleen, sepsis |
| Sodium | About 135–145 mmol/L | Water loss, dehydration | Excess water retention, some medicines, heart or liver failure |
| Potassium | About 3.5–5.0 mmol/L | Kidney failure, some medicines, cell breakdown | Vomiting, diarrhea, diuretics |
| Creatinine | Roughly 0.6–1.3 mg/dL, depending on muscle mass | Reduced kidney function | Low muscle mass |
| Glucose (fasting) | About 70–99 mg/dL (3.9–5.5 mmol/L) | Diabetes, stress, steroids | Excess insulin or diabetes medicines, alcohol, liver failure |
| Calcium (total) | About 8.5–10.5 mg/dL (2.1–2.6 mmol/L) | Overactive parathyroid glands, some cancers | Underactive parathyroid glands, vitamin D deficiency, severe pancreatitis |
Kidney tests
Creatinine is a waste product of muscle that the kidneys clear from the blood, so it rises when kidney function falls. Labs often convert it into an estimated glomerular filtration rate (eGFR). Urea (blood urea nitrogen, or BUN) also rises with kidney problems, and with dehydration or bleeding in the gut.
Liver tests
"Liver function tests" are really a mix of markers. ALT and AST are enzymes released when liver cells are damaged, so they rise sharply in hepatitis. Alkaline phosphatase (ALP) and bilirubin rise when bile flow is blocked, for example by a gallstone in the bile duct. Albumin and clotting tests reflect how well the liver makes proteins. The pattern of results usually matters more than any single value.
Cardiac markers
Troponin is a protein released when heart muscle cells are injured. A value above the laboratory's upper reference limit (set at the 99th percentile of healthy people) shows injury, and a rise or fall over a few hours shows the injury is recent. A heart attack (myocardial infarction) needs that pattern plus evidence that the injury was caused by reduced blood supply, such as typical chest pain or ECG changes. Troponin also rises in myocarditis, pulmonary embolism, sepsis, heart failure and kidney disease, which is why it has to be read alongside the rest of the case.
Inflammation markers
C-reactive protein (CRP) and the erythrocyte sedimentation rate (ESR) rise with infection and inflammation of almost any kind. They are useful for tracking whether a condition is improving, but they rarely tell you what the condition is.
Pancreas and thyroid
Lipase rises in acute pancreatitis. One common definition of acute pancreatitis requires two of three features: typical upper abdominal pain, a lipase (or amylase) at least three times the upper limit of normal, and characteristic imaging. For the thyroid, TSH is the usual first test: in the most common forms of thyroid disease it is high when the thyroid is underactive and low when it is overactive, because the pituitary gland turns TSH up or down in response.
Reading a result, step by step
When a result comes back flagged, it helps to ask:
- How far outside the range is it? Slightly or dramatically?
- Does it fit the clinical picture? Does it support one of your possible diagnoses?
- Has it changed? Compare it with earlier results if there are any.
- Could it be an artifact? A potassium can read falsely high if blood cells burst in the sample tube, for example.
- What will you do differently because of it? If nothing, it may not have needed testing.
Labs in Stat!
In Syndrome, the Labs tab lets you order blood and other laboratory tests and see how each one came back. The best players treat labs the way clinicians do: they order the test that would best separate their top candidate diagnoses, and they give a normal result as much weight as an abnormal one. Our Syndrome guide has more strategy.
Stat! and this guide are for education only. They are not medical advice and not a substitute for professional care. If you have a health concern, talk to a qualified clinician.