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How to Read a Blood Test
A plain-language, educational guide to what a blood test shows — what a CBC and a basic metabolic panel measure, how reference ranges work, and what an "out of range" result generally means.
Two common panels
Most routine blood draws include one or both of these:
1. Complete Blood Count (CBC)
A CBC looks at the cells in your blood: red blood cells (which carry oxygen — often summarised by hemoglobin and hematocrit), white blood cells (part of your immune defence), and platelets (which help blood clot). It is one of the most frequently ordered tests.
2. Basic / Comprehensive Metabolic Panel
A metabolic panel measures chemistry in the fluid part of your blood — for example glucose (blood sugar), electrolytes such as sodium and potassium, and markers related to kidney function (e.g. creatinine). A comprehensive panel adds liver-related markers and proteins.
How to read the columns
A typical lab report has a few columns next to each marker:
- Your result — the measured value.
- Units — e.g. mg/dL, g/dL, mmol/L. The same marker can be reported in different units in different countries, so the number only makes sense next to its unit.
- Reference range — the range considered typical for a healthy population at that specific laboratory. Results outside it are often flagged "H" (high) or "L" (low).
What is a reference range?
A reference range is usually defined so that about 95% of healthy people fall inside it. By design, a small share of perfectly healthy people sit just outside. Ranges differ between laboratories and depend on factors such as age and sex — which is exactly why you should read your result against the range printed on your report, not a number you found online.
What does "out of range" mean?
An out-of-range value is a signal to look closer, not a diagnosis. A result can fall outside the range for many ordinary reasons:
- Recent food, drink, caffeine or exercise (some tests require fasting).
- Hydration, time of day, or medications and supplements.
- Normal lab-to-lab and day-to-day variation.
- Your individual baseline simply differing from the population average.
Clinicians interpret a result in context — alongside your symptoms, history, physical exam, trends over time, and other markers. A single flagged value, especially one only slightly outside the range, is often not concerning on its own.
A sensible way to approach your results
- Note which values are flagged and by how much.
- Check whether the test required fasting or special preparation.
- Look at trends if you have past results.
- Bring questions to your clinician — they can tell you what your numbers mean for you.
Want a marker-by-marker explainer?
See our companion page on common markers with widely-accepted reference ranges, each cited to an authoritative medical source.