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Patient Guide

How to Read a Blood Test Report

A blood report looks confusing at first: rows of numbers, abbreviations and ranges. This guide explains every part of the report so you can understand it before seeing your doctor.

Written by Suman Konda, PharmD, Clinical Pharmacist · Editorial policy · Not medical advice

Last updated: · How we check our content

The anatomy of a blood report

A typical lab report has these columns:

ColumnWhat it means
Test NameThe name of what was measured (e.g., Haemoglobin, TSH, Creatinine)
Result / ValueYour actual measured value
UnitThe measurement unit (g/dL, mg/dL, IU/L, pg/mL, mIU/L, etc.)
Reference RangeThe normal range for that test (varies by lab, age and gender)
H / L / * flagH = your result is Higher than normal. L = Lower than normal. * = abnormal

What do the units mean?

UnitStands forCommonly used for
g/dLGrams per decilitreHaemoglobin
mg/dLMilligrams per decilitreBlood sugar, cholesterol, creatinine, uric acid
µg/dLMicrograms per decilitreSerum iron, TIBC
ng/mLNanograms per millilitreFerritin, PSA, Vitamin D
pg/mLPicograms per millilitreVitamin B12, T3, T4
mIU/L or µIU/mLMilli/micro International Units per litre/mLTSH, FSH, LH, insulin
IU/L or U/LInternational Units per litreLiver enzymes (SGPT, SGOT, ALP)
/µL or cells/mm³Per microlitreWBC, RBC, platelet counts
%PercentageHbA1c, haematocrit, transferrin saturation
mm/hrMillimetres per hourESR

What does it mean when a value is flagged H or L?

H (High) flag

Your result is above the upper limit of the reference range. This does not automatically mean something is seriously wrong. How much above normal, your symptoms, and what other tests show all matter. A mildly elevated value may be insignificant; a very high value may need urgent attention. Context is everything.

L (Low) flag

Your result is below the lower limit of the reference range. Again, mildly low values are often not clinically significant on their own. Low haemoglobin with symptoms = anaemia; borderline low B12 with no symptoms may just need dietary improvement.

Reference ranges: why they vary

Reference ranges differ between labs because they are calculated from the lab's own population of healthy volunteers. This is why a result marked normal in one lab may be flagged abnormal in another. Always compare your result to the reference range printed on YOUR report, not ranges from the internet. Age, gender and pregnancy also shift normal ranges.

How to use this report with your doctor

  1. Note every value flagged H or L
  2. Note by how much it's outside the range (slightly vs very)
  3. Write down your symptoms and when they started
  4. Ask: "Is this level clinically significant given my symptoms?"
  5. Ask: "Do I need a follow-up test or can we watch and recheck?"
  6. Don't self-diagnose: patterns across multiple tests matter more than single values

Explore specific tests

Click any test below to understand its normal ranges and what high or low values mean:

Related reading

Medical Disclaimer: This page is for general educational purposes only and does not constitute medical advice. Always consult a qualified doctor before making any health decisions.