A measurand is the specific quantity you intend to measure. It’s not the substance itself, but the particular property of that substance you’re quantifying. The term comes from metrology, the science of measurement, and it matters because the same substance can be measured in different ways, each producing a different result. Understanding what a measurand is helps explain why precision in measurement descriptions is so important, especially in fields like laboratory medicine and engineering.
Measurand vs. Analyte
The easiest way to understand a measurand is to compare it with a related term: analyte. An analyte is simply the substance you’re interested in. The measurand is the specific property of that substance your measurement system actually quantifies.
Take sodium as an example. Sodium is the analyte. But you could measure sodium in several ways: the concentration of sodium in urine, the concentration of sodium in plasma, or the activity concentration of sodium in plasma. Each of these is a different measurand, even though the analyte (sodium) is the same every time. The distinction is practical. Two labs could both be “measuring sodium” but actually be measuring different measurands, which means their results aren’t directly comparable unless you know exactly what each lab quantified.
Here are a few more examples from clinical laboratories:
- Analyte: creatine kinase MB. One measurand could be its concentration in plasma. Another could be its activity concentration in plasma. Same substance, different quantities.
- Analyte: alkaline phosphatase. The measurand is typically its activity concentration in plasma, meaning you’re measuring how much enzymatic work it does, not just how much of it is present.
- Analyte: glucose. The measurand might be the mass concentration of glucose in serum, measured in milligrams per deciliter, or the substance concentration in millimoles per liter.
What Makes a Complete Measurand Description
A measurand isn’t fully defined by just naming a substance and a property. According to the International Bureau of Weights and Measures (BIPM), specifying a measurand requires knowing the kind of quantity (concentration, activity, mass, voltage, etc.), a description of the body or substance carrying that quantity, and any relevant components or chemical entities involved. In clinical testing, this typically means stating the sample type (plasma, serum, urine), the substance being measured, and the kind of quantity (concentration, activity concentration, and so on).
For laboratory accreditation under ISO 15189, this level of specificity is built into how labs define their scope. Tests are grouped by the measurand, the medical field (clinical chemistry, hematology), the type of measurand (enzymes, hormones, biomarkers), the analytical method used, and the sample type. Each grouping depends on the measurand being clearly defined so that competence can be assessed and results can be trusted across different laboratories.
Why Defining It Precisely Matters
An incompletely defined measurand introduces uncertainty into your measurement, and sometimes that uncertainty is the largest source of error. The Joint Committee for Guides in Metrology puts it this way: in principle, a measurand cannot be completely described without an infinite amount of information. Every detail you leave out creates room for interpretation, which creates room for variation in results.
Consider measuring the thickness of a sheet of material with a micrometer. “Thickness of the sheet” sounds like a clear measurand, but it doesn’t specify where on the sheet to measure. Different locations will give different values, each technically correct given the definition. The spread of those values represents uncertainty that comes purely from the measurand being incompletely defined. Barometric pressure, humidity, how the sheet is supported, its orientation in gravity: all of these could affect the result, and none of them are addressed by a vague description. To reduce uncertainty, you need a more complete definition of what you’re measuring.
This is also why two laboratories can measure “the same thing” and get different results. If their measurand definitions differ even slightly, say one measures total drug concentration in plasma while the other measures only the free (unbound) fraction, their numbers will disagree. Neither lab is wrong. They’re measuring different measurands.
Measurands in Drug Monitoring
Therapeutic drug monitoring is a good real-world example of why measurand definition matters for patient care. When a doctor orders a blood level for a medication, the lab needs to know exactly what to measure. For drugs that produce active metabolites, both the parent drug and the metabolites may need to be measured to get a meaningful picture of total active compound concentration. That total concentration is the measurand.
But total drug concentration doesn’t always tell the whole story. Factors like protein binding, electrolyte balance, acid-base status, and age can all change how much of the drug is actually active in the body. If the measurand is total plasma concentration, these factors become what metrologists call influence quantities: things that don’t change the number the instrument reads but do change the relationship between that number and the clinical reality it’s supposed to represent. Choosing the right measurand (total vs. free drug concentration, for instance) determines whether the lab result is clinically useful.
Influence Quantities and the Measurand
Not everything that affects a measurement is part of the measurand. Influence quantities are factors that affect the measurement system or the relationship between what the instrument reads and the final result, without being the thing you’re actually trying to measure. The temperature of a micrometer affects its readings, but it’s not part of the measurand (the thickness of the object). By contrast, the temperature of the object itself could be part of the measurand’s definition, since materials expand and contract with temperature changes.
In clinical chemistry, bilirubin concentration in a blood sample can interfere with a hemoglobin measurement. The bilirubin isn’t what you’re trying to measure, but it influences the result. Recognizing these influence quantities and separating them from the measurand is essential for understanding where errors come from and how to correct for them.
The Core Idea
A measurand is the bridge between “what substance am I interested in?” and “what number does the instrument give me?” It forces you to be specific about exactly which property, in which sample, under which conditions, you’re quantifying. The more precisely you define it, the less ambiguity there is in your result, and the more confidently that result can be compared to measurements made elsewhere. In everyday terms, it’s the difference between saying “check my blood sugar” and specifying “measure the concentration of glucose in my fasting venous plasma in millimoles per liter.” Both point to the same substance, but only the second one is a fully defined measurand.