Blood Test Units Explained: mmol/L, g/L, U/L & More
Your blood test report is full of unfamiliar units — mmol/L, µmol/L, g/L, U/L, nmol/L. Here's what every unit means in plain language, how to convert between international systems, and why different labs sometimes report different units for the same marker.
Every Common Blood Test Unit Decoded
Australia uses SI (Système International) units for pathology, which is the global scientific standard. Here is every unit you will encounter on an Australian blood test report:
| Unit | Full Name | Common Markers | Plain Explanation |
|---|---|---|---|
| mmol/L | Millimoles per litre | Glucose, cholesterol (total, HDL, LDL), triglycerides, sodium, potassium, chloride, bicarbonate, urea, calcium, magnesium | The number of molecular "units" of a substance in one litre of blood. This is the standard SI unit used in Australia for most common chemistry markers. One mmol is one-thousandth of a mole. |
| µmol/L | Micromoles per litre | Creatinine, bilirubin, urate (uric acid), iron, copper, zinc | Same concept as mmol/L but 1000 times smaller. Used for substances that are present in much lower concentrations. One µmol is one-millionth of a mole. If a marker is in µmol/L, its concentration is much lower than one measured in mmol/L. |
| nmol/L | Nanomoles per litre | Vitamin D (25-OH), testosterone, cortisol, progesterone, estradiol (some labs), DHEA-S | One billion times smaller than a mole. Used for hormones and vitamins that exist in tiny concentrations in the blood. A vitamin D level of 75 nmol/L means there are 75 nanomoles of vitamin D per litre of blood. |
| pmol/L | Picomoles per litre | Free T4 (thyroxine), free T3, estradiol (some labs), vitamin B12, folate | One trillion times smaller than a mole. Used for substances present in vanishingly small amounts. Thyroid hormones circulate in picomolar concentrations because even tiny changes have large physiological effects. |
| g/L | Grams per litre | Haemoglobin, albumin, total protein, immunoglobulins (IgG, IgA, IgM), fibrinogen | The weight of a substance in one litre of blood. Used for proteins and other large molecules where counting individual molecules is impractical. In Australia, haemoglobin is reported as g/L (e.g., 140 g/L). |
| g/dL | Grams per decilitre | Haemoglobin (US/UK), albumin (some international labs) | Common in the United States and some UK labs. To convert: g/dL × 10 = g/L. So a US haemoglobin of 14.0 g/dL equals the Australian value of 140 g/L. If your results use g/dL, you are likely looking at an international report. |
| U/L | Units per litre (enzyme units) | ALT, AST, GGT, ALP (alkaline phosphatase), LDH, CK (creatine kinase), lipase, amylase | Measures enzyme activity, not concentration. One "unit" is the amount of enzyme that catalyses a specific reaction at a defined rate. Higher U/L means more enzyme activity, which usually means more of that enzyme has been released from damaged cells (e.g., liver damage releases ALT). |
| IU/L | International Units per litre | Ferritin, vitamin B12 (some labs), folate (some labs), insulin, PSA (some labs) | Similar to U/L but standardised by the WHO for biological substances where activity varies between preparations. One IU of vitamin D has a defined biological effect regardless of the molecular form. Not interchangeable with U/L for the same marker. |
| ×10⁹/L | Times ten to the ninth per litre (giga per litre) | White blood cell count (WCC), platelet count, neutrophils, lymphocytes, monocytes, eosinophils, basophils | The number of cells in one litre of blood, expressed in billions. A WCC of 7.0 ×10⁹/L means 7 billion white cells per litre. This notation avoids writing enormous numbers. The US uses the equivalent ×10³/µL (thousands per microlitre) — the numerical value is the same. |
| ×10¹²/L | Times ten to the twelfth per litre (tera per litre) | Red blood cell count (RCC/RBC) | The number of red blood cells per litre, in trillions. A normal RBC of 5.0 ×10¹²/L means 5 trillion red cells per litre. Red cells are far more numerous than white cells, so they need a larger unit. |
| mIU/L | Milli-international units per litre | TSH, FSH, LH, prolactin, insulin (some labs) | One thousandth of an International Unit per litre. Used for hormones where the biologically active amount is very small. A TSH of 2.5 mIU/L is a standard normal value. Some labs report TSH as mU/L or µIU/mL — these are numerically equivalent. |
| µg/L | Micrograms per litre | Ferritin, PSA, troponin, procalcitonin, tumour markers (CEA, CA-125, AFP) | The weight of a substance in micrograms per litre. One microgram is one millionth of a gram. Used for substances present in very small quantities where molecular weight makes molar units impractical. Ferritin of 50 µg/L means 50 millionths of a gram per litre. |
SI vs Conventional Units: Converting Between Systems
Australia, Europe, and most of the world use SI units. The United States uses conventional units (also called "traditional" or "US customary" units). If you are comparing your Australian results with US-based reference material, you need to convert. Here are the most common conversions:
| Marker | AU Unit (SI) | US Unit | Conversion | Example |
|---|---|---|---|---|
| Glucose | mmol/L | mg/dL | mmol/L × 18 = mg/dL | 5.5 mmol/L = 99 mg/dL |
| Cholesterol (Total) | mmol/L | mg/dL | mmol/L × 38.67 = mg/dL | 5.0 mmol/L = 193 mg/dL |
| Haemoglobin | g/L | g/dL | g/L ÷ 10 = g/dL | 140 g/L = 14.0 g/dL |
| Creatinine | µmol/L | mg/dL | µmol/L ÷ 88.4 = mg/dL | 90 µmol/L = 1.02 mg/dL |
| Bilirubin | µmol/L | mg/dL | µmol/L ÷ 17.1 = mg/dL | 15 µmol/L = 0.88 mg/dL |
| Urate (Uric Acid) | mmol/L | mg/dL | mmol/L × 16.81 = mg/dL | 0.35 mmol/L = 5.9 mg/dL |
| Testosterone | nmol/L | ng/dL | nmol/L × 28.84 = ng/dL | 15 nmol/L = 433 ng/dL |
| Vitamin D | nmol/L | ng/mL | nmol/L ÷ 2.496 = ng/mL | 75 nmol/L = 30 ng/mL |
| Iron | µmol/L | µg/dL | µmol/L × 5.585 = µg/dL | 15 µmol/L = 83.8 µg/dL |
| HbA1c | mmol/mol (IFCC) | % (DCCT) | IFCC = (DCCT − 2.15) × 10.929 | 48 mmol/mol = 6.5% |
The "H" and "L" Flags on Your Report
Pathology reports flag results that fall outside the laboratory's reference range. Understanding these flags helps you read your report without unnecessary alarm:
H or HI — Result is above the upper limit of the reference range (High)
Does not automatically mean disease. Mildly elevated results can be normal variation, diet-related, or medication effects. Significantly elevated results need GP review.
L or LO — Result is below the lower limit of the reference range (Low)
Same principle — mildly low may be insignificant, very low usually needs investigation. Your GP will assess the clinical context.
HH or CRIT (High Critical) — Result is critically high and may need urgent action
The lab will contact your GP directly for critical values. If you see this flag, your doctor has likely already been notified. Contact your GP urgently if you haven’t heard from them.
LL or CRIT (Low Critical) — Result is critically low and may need urgent action
Same as above — the lab contacts the ordering doctor directly. Urgently follow up with your GP.
* (asterisk) — Result is outside the reference range (some labs use this instead of H/L)
Check whether the result is above or below the range. An asterisk simply flags an out-of-range result.
No flag — Result is within the reference range
Generally reassuring, but "within range" does not always mean optimal. Some markers (e.g., ferritin, vitamin D) have "adequate" levels within range but "optimal" levels at the higher end.
Common Sources of Confusion
Same marker, different units from different labs
Australian labs overwhelmingly use SI units, but if you are comparing with US results, overseas reports, or online references, the units may differ. Always check the unit before comparing numbers. A glucose of 5.5 (mmol/L) is normal; a glucose of 5.5 (mg/dL) would be critically low (the correct US equivalent is ~99 mg/dL).
HbA1c in mmol/mol vs percentage (%)
Australia uses the IFCC standard (mmol/mol), while the US uses the DCCT standard (%). A normal HbA1c is <42 mmol/mol or <6.0%. The diabetes diagnostic threshold is ≥48 mmol/mol or ≥6.5%. Most pathology reports now show both values, but older reports may only show one.
Reference ranges differ between labs
Each pathology laboratory sets its own reference ranges based on their specific testing equipment, methods, and local population data. A "normal" TSH at Laverty might be 0.5–4.0 mIU/L while Sullivan Nicolaides uses 0.4–4.5 mIU/L. Always compare your result to the range printed on YOUR report, not ranges from the internet.
U/L vs IU/L for the same marker
Some markers (like ferritin) are reported as µg/L by some labs and IU/L or ng/mL by others. The numerical values may or may not be the same depending on the assay. When tracking a marker over time, try to use the same laboratory for consistency.
Why "normal" doesn’t mean "optimal"
Reference ranges define the range that 95% of healthy people fall within. A ferritin of 15 µg/L is technically "in range" but represents borderline iron stores that many clinicians would treat. Similarly, a vitamin D of 51 nmol/L is "adequate" but many experts recommend 75+ nmol/L. The H/L flags are starting points, not the complete picture.
How SmarterBlood Handles Units
One of the most frustrating aspects of tracking your health over time is dealing with inconsistent units across different pathology reports. SmarterBlood solves this automatically:
Automatic unit detection
Our AI reads the units directly from your pathology PDF. Whether your lab reports haemoglobin as g/L (Australia) or g/dL (US/UK), SmarterBlood identifies and records the correct unit.
Unit normalisation for trending
When you upload results from different labs or countries, SmarterBlood converts all values to a consistent unit system for accurate trend graphs. Your glucose from 2020 (US report in mg/dL) and 2026 (Australian report in mmol/L) will appear on the same graph correctly.
Reference range alignment
SmarterBlood uses RCPA (Royal College of Pathologists of Australasia) master reference ranges as the baseline, adjusted for age and sex when available. We flag values against these standardised ranges rather than relying solely on individual lab ranges.
Cross-lab comparison safety
When markers from different labs use different units, SmarterBlood converts before comparing and clearly shows the original unit alongside the normalised value. You never accidentally compare 140 g/L with 14.0 g/dL and think they are different.
Related Reading
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Unit information sourced from the Royal College of Pathologists of Australasia (RCPA), International System of Units (SI), and Australian pathology laboratory standards. Conversion factors are standard and verified. SmarterBlood provides health information and AI-powered blood test analysis. It is not a substitute for professional medical advice, diagnosis, or treatment.