Clinical laboratories rely on three main measurement systems: the International System of Units (SI), conventional units, and International Units (IU) for biological activity. Which system appears on a lab report depends on the country, the specific test, and the substance being measured. In the United States, most results still use conventional units, while laboratories in Europe, Canada, and Australia report primarily in SI units.
The International System of Units (SI)
The SI system is the global standard for scientific measurement, built on a small set of base units. In laboratory medicine, four base units do most of the heavy lifting: the metre (m) for length, the kilogram (kg) for mass, the second (s) for time, and the mole (mol) for amount of substance. The International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) and the International Union of Pure and Applied Chemistry (IUPAC) jointly recommend SI-based reporting for lab results worldwide.
What makes SI units distinctive in clinical chemistry is the preference for molar concentrations. Instead of reporting how much a substance weighs in a given volume of blood, SI expresses how many molecules are present. Glucose reported as 5.5 mmol/L, for instance, tells you the number of glucose molecules per liter of blood. This approach is more useful for understanding chemical reactions in the body, because biological processes happen molecule by molecule, not gram by gram.
Conventional Units
Conventional units are the older system still dominant in U.S. laboratories. They typically express concentrations as mass per volume: milligrams per deciliter (mg/dL) or grams per deciliter (g/dL). If you’ve ever seen a blood glucose result of 100 mg/dL or a hemoglobin of 14 g/dL, you were reading conventional units.
The two systems measure the same thing but express it differently, and converting between them requires analyte-specific multiplication factors. A few common examples illustrate how much the conversion factors vary:
- Glucose: Multiply mg/dL by 0.0555 to get mmol/L. A conventional reading of 100 mg/dL equals roughly 5.6 mmol/L.
- Cholesterol: Multiply mg/dL by 0.0259 to get mmol/L. A reading of 200 mg/dL equals about 5.2 mmol/L.
- Creatinine: Multiply mg/dL by 88.4 to get µmol/L. A reading of 1.0 mg/dL equals 88.4 µmol/L.
- Hemoglobin: Multiply g/dL by 10 to get g/L. A reading of 15 g/dL equals 150 g/L.
- Thyroxine (T4): Multiply µg/dL by 12.9 to get nmol/L.
Some analytes convert at a 1:1 ratio. Potassium, for example, is 1 mEq/L in conventional units and 1 mmol/L in SI, so the number stays the same. Others, like creatinine, shift by a factor of nearly 90, which is why mixing up systems can cause confusion or even clinical errors.
Metric Prefixes for Very Small Quantities
Both SI and conventional units use metric prefixes to handle the extremely small concentrations found in blood and body fluids. The prefixes you’ll encounter most often in lab work are milli (one-thousandth), micro (one-millionth), nano (one-billionth), and pico (one-trillionth). A hormone like thyroid-stimulating hormone might be reported in micro-International Units per milliliter, while a tumor marker could appear in nanograms per milliliter. Vitamin D metabolites sometimes reach into the picomole range.
Volume measurements follow the same scaling. Blood samples are collected in milliliters (mL), but when a test requires only a tiny drop, the measurement drops to microliters (µL). Knowing the prefix hierarchy helps you read lab results without confusing a microgram for a milligram, a thousandfold difference that matters enormously.
International Units for Biological Activity
Some substances can’t be meaningfully measured by weight alone because their biological potency varies from batch to batch. Hormones, vitamins, vaccines, and clotting factors fall into this category. For these, laboratories use International Units (IU), a system maintained by the World Health Organization.
An IU is defined by how much biological effect a substance produces, not by how much it weighs. The WHO maintains physical reference standards, stored in ampoules, that labs and manufacturers calibrate against. One IU of vitamin D3, for example, equals 25 nanograms of the compound. For erythropoietin (EPO), the current WHO reference standard assigns 1,650 IU to approximately 11 micrograms of the recombinant protein. The relationship between IU and mass is unique to each substance, so there is no single conversion factor that works across the board.
You’ll see IU on labels for vitamin supplements, insulin, and certain injectable medications. In lab reports, IU often appears for hormone levels and enzyme activity.
Hematology: Cell Counts and Blood Values
Blood cell counts use their own reporting conventions. In conventional terms, a normal white blood cell count falls between 4,500 and 11,000 cells per cubic millimeter (mm³). In SI, the same range is expressed as 4.5 to 11.0 × 10⁹ per liter. Red blood cells are reported in millions per mm³ conventionally, or as × 10¹² per liter in SI. Normal values for men range from 4.3 to 5.9 million/mm³ and for women from 3.5 to 5.5 million/mm³.
Hemoglobin is reported as a concentration. Conventional units use grams per deciliter (normal range: 13.5 to 17.5 g/dL for men, 12.0 to 16.0 g/dL for women), while SI units express it as millimoles per liter (2.09 to 2.71 mmol/L for men, 1.86 to 2.48 mmol/L for women). Mean corpuscular volume (MCV), which measures the average size of a red blood cell, is one value that reads identically in both systems: the old unit of cubic micrometers (µ³) is numerically equal to the SI unit of femtoliters (fL).
Titers in Immunology
Antibody testing uses a measurement approach that looks nothing like the systems above. Instead of a concentration, results are often reported as a titer: the highest dilution of a patient’s blood that still shows a detectable immune reaction. A titer of 1:64 means the sample was diluted 64-fold and still tested positive. A titer of 1:256 indicates a stronger antibody response because the sample could be diluted much further before losing detectability.
Titers are determined through assays that mix the patient’s serum with known target molecules and look for visible clumping, color changes, or other signs of an antibody binding to its target. These results guide decisions about vaccination boosters, help diagnose active or past infections, and monitor autoimmune conditions. Because titers are ratios rather than absolute concentrations, they don’t carry traditional units like mg/dL or mmol/L.
Temperature and Incubation Standards
Temperature measurement in the lab uses the Celsius scale for nearly all practical purposes: storing reagents, incubating cultures, and calibrating instruments. The Celsius degree is defined as equal in magnitude to the kelvin, the SI base unit for thermodynamic temperature, with a simple offset of 273.15 (so 37°C equals 310.15 K). Laboratories follow the International Temperature Scale of 1990 (ITS-90) for calibration, which uses the freezing, melting, and triple points of specific reference materials as checkpoints to ensure thermometers read accurately.
In daily lab work, you’ll see Celsius on incubator displays, refrigerator logs, and test protocols. Kelvin occasionally appears in instrument specifications or quality-control documentation, but Celsius dominates the bench.
Why Multiple Systems Persist
The coexistence of SI and conventional units is largely a matter of clinical habit. U.S. physicians trained on conventional values carry decades of pattern recognition. A doctor who knows that normal fasting glucose is around 70 to 100 mg/dL would need to retrain that instinct to think in the SI equivalent of 3.9 to 5.6 mmol/L. Multiply that retraining across every analyte and every clinician, and the cost of switching becomes enormous.
Most laboratory information systems can display results in either format, and reference ranges are always paired with the units on the report. If you’re comparing results from different countries or different labs, checking which unit system was used is the single most important step before interpreting the numbers.