Patient Guide

How to Read a Blood Test Report

Name: How to Read a Blood Test Report | HealthReview
Creator: Suman Konda
Published: 2026-07-05

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.¹

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

Last updated: 01 August 2026 · How we check our content

The anatomy of a blood report

A typical lab report has these columns:

Column	What it means
Test Name	The name of what was measured (e.g., Haemoglobin, TSH, Creatinine)
Result / Value	Your actual measured value
Unit	The measurement unit (g/dL, mg/dL, IU/L, pg/mL, mIU/L, etc.)
Reference Range	The normal range for that test (varies by lab, age and gender)
*H / L / flag**	H = your result is Higher than normal. L = Lower than normal. * = abnormal

What do the units mean?

Unit	Stands for	Commonly used for
g/dL	Grams per decilitre	Haemoglobin
mg/dL	Milligrams per decilitre	Blood sugar, cholesterol, creatinine, uric acid
µg/dL	Micrograms per decilitre	Serum iron, TIBC
ng/mL	Nanograms per millilitre	Ferritin, PSA, Vitamin D
pg/mL	Picograms per millilitre	Vitamin B12, T3, T4
mIU/L or µIU/mL	Milli/micro International Units per litre/mL	TSH, FSH, LH, insulin
IU/L or U/L	International Units per litre	Liver enzymes (SGPT, SGOT, ALP)
/µL or cells/mm³	Per microlitre	WBC, RBC, platelet counts
%	Percentage	HbA1c, haematocrit, transferrin saturation
mm/hr	Millimetres per hour	ESR

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

Note every value flagged H or L
Note by how much it's outside the range (slightly vs very)
Write down your symptoms and when they started
Ask: "Is this level clinically significant given my symptoms?"
Ask: "Do I need a follow-up test or can we watch and recheck?"
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:

References

Sources cited on this page. PubMed links open the original abstract.

Lippi G, Plebani M. The importance of intra-laboratory turnaround time for laboratory diagnostics. Clin Biochem. 2011;44(5-6):490–492. PMID 21172328 · doi:10.1016/j.clinbiochem.2010.12.001

Understanding reference ranges – they are not absolute truth

Every blood test report includes a reference range – the interval within which 95% of healthy people fall. This is defined statistically: the range is set to include 95% of a normal population sample, which means 5% of healthy people will fall outside the range by definition. A result "outside normal" therefore has at least a 5% chance of being a statistical outlier in a perfectly healthy person.

Critical points about reference ranges:

Reference ranges vary between laboratories (different analysers, different calibration methods). A result of 41 IU/L might be within range at one lab (upper limit 42) and flagged as abnormal at another (upper limit 40). Always interpret against the specific lab's range on the report.
Reference ranges are population averages and may not apply to specific subgroups. Creatinine reference ranges are based on average muscle mass – a very muscular person will have a "high normal" creatinine that is normal for them; a frail elderly person may have a "normal" creatinine that actually masks significant kidney impairment because muscle mass is low.
A result within the reference range is "normal" – but "normal" is not the same as "optimal for this patient." An HbA1c of 46 mmol/mol is within the non-diabetic range but indicates pre-diabetes (42–47 mmol/mol), where lifestyle intervention can prevent progression to diabetes.

How to read a blood report systematically

A structured approach to reviewing a blood report:

Identify the test and its unit: Units matter enormously. Glucose of 7.0 mmol/L (normal fasting) is very different from 7.0 mg/dL (critically low). Unit errors have caused fatal dosing mistakes.
Check the date and time of collection: Cortisol must be interpreted against collection time (peak at 9 am, nadir at midnight). PSA may be transiently elevated after urinary tract instrumentation or vigorous exercise.
Compare with previous results: The trend matters more than any single value. A creatinine rising from 70 to 100 µmol/L – both within reference range – is a 40% increase that may signal early CKD. A rising PSA, even within range, warrants monitoring.
Look for patterns across multiple tests: An isolated raised ALT may be benign; raised ALT + raised ALP + raised bilirubin = cholestatic jaundice requiring imaging. Low sodium + low potassium + low creatinine = diuretic effect. High calcium + low phosphate + high PTH = primary hyperparathyroidism.
Consider the clinical context: A CRP of 80 mg/L in a post-operative patient on day 2 is expected; the same CRP in a person with no known acute illness demands explanation.