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Blood and lab tests

What Do H and L Mean on a Blood Test Report?

High and Low Flags (H and L) · say it: high and low flagsAlso written: H, L, abnormal flag, out of range flag

Short answer

An H or L next to a lab result means the value is above (high) or below (low) the reference range the lab printed on your report.

What it means

Many lab reports add a letter or symbol next to results that fall outside the reference range. H usually means high, L means low, and some labs use arrows, asterisks or bold text instead.

The flag is applied automatically by the lab's computer. It does not judge how much the result matters; it only says the number sits outside the lab's usual band.

Some reports also use special markers such as "critical" or "panic" for results the lab thinks a clinician should see quickly. Your report may explain its own symbols at the bottom of the page.

Where you may see it

  • Blood count and chemistry reports
  • Patient portals and app summaries
  • Printed results handed out at clinics

📄 How it looks on a report

Potassium 5.4 mmol/L H

🙅 What it does not mean

  • A flag does not automatically mean you are ill. Healthy people often have one or two flagged results on a long panel.
  • No flag does not mean everything has been checked; a doctor may still look at trends or combinations of results.
  • H and L do not tell you how serious a result is. A value just over the line and one far outside it get the same letter.

🗒️ Questions to ask your doctor

🧾 Reports where you will see “High and Low Flags (H and L)”

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🧩 Related words

Reference RangeWithin Normal LimitsBorderlineHemolyzed Sample

🔎 Look up another word

More words: blood and lab tests

ALT (Alanine Aminotransferase)AST (Aspartate Aminotransferase)BilirubinCreatinineCRP (C-Reactive Protein)Culture and SensitivityeGFR (Estimated Glomerular Filtration Rate)ElectrolytesESR (Erythrocyte Sedimentation Rate)Fasting SampleFerritinHbA1cAll medical words →
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General information about a medical word, not advice about your own results. Ask the clinician who ordered the test what it means for you.