How to Read Blood Test Results
Content last reviewed 2026-09-12 · Educational information — not medical advice
Quick answer
Read three things in order: YOUR value, the lab's printed reference interval beside it, and the flag (H/L). Then judge degree (barely outside vs multiples of limit) and trend (vs your prior results) — never the number alone.
The anatomy of a lab report
Each line shows: test name, your value + units, reference interval, and often a flag. Units differ internationally (mg/dL vs mmol/L) — never compare numbers across unit systems. The interval reflects YOUR laboratory's instrument and population, not a universal constant.
What flags actually mean
High and Low markers denote outside-interval values — information, not verdicts. Borderline excursions (within 10–20% of the edge) behave differently than dramatic ones. Laboratories auto-flag; clinicians interpret significance.
Reference ranges are statistical, not moral
Intervals typically capture the central 95% of healthy populations — meaning 1-in-20 healthy people fall outside by definition. Single borderline values in asymptomatic people frequently just get repeated.
Trends outrank snapshots
Your creatinine at 1.1 stable for five years tells a different story than 1.1 newly risen from 0.8. Keep your own results file; comparison transforms interpretation from guesswork to trajectory reading.
Where to go deeper
Our interpretation guides break down what high and low values mean for 49 individual markers — from ALT to ferritin to TSH — in plain language.
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Frequently asked questions
Can AI interpret my labs?
AI tools explain patterns and context well — they complement but don't replace clinical correlation with your history, exam, and medications.
What's the most commonly misunderstood line?
Probably 'out of range' being read as 'disease'. Ranges describe population statistics; your context determines whether deviation matters.