General rules on measurement unit
Purpose |
A quantitative estimation of a clinical laboratory analyte must be expressed together with an appropriate unit of measurement. Without units, the result is at best useless, at worst dangerously misleading and a great risk to patient care. Many potential forms of measurement unit are available for a given estimation, however the result will be of most benefit to the patient and clinician if the unit is scientifically relevant and commonly employed by local laboratories. | ||
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Recommendation
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The International Union of Pure and Applied Chemistry (IUPAC) publishes recommendations for terminology and nomenclature in clinical laboratory medicine in the “Silver Book” (ref. 1). The International System of Units (“SI units” – Systeme Internationale) offers measurement units based on physical quantities and are normally preferable for laboratory estimations in most parts of the world. | ||
Patient safety
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Estimations expressed with no unit create risks for patient management. The recipient of the estimation may assume an incorrect unit, with consequent misinterpretation of the result against a reference or therapeutic interval. Therapeutic drug estimations are an example of how dangerous such misinterpretations may be. A digoxin result of 1.0 nmol/L communicated without units might be assumed to represent a value of 1.0 in mass concentration units (e.g. µg/L). In fact a digoxin value of 1.0 nmol/L is equivalent to a value of 0.8 µg/L. A 20% difference is small enough to increase the risk of error of interpretation not being detected but large enough to risk patient health (e.g. if clinician elected to increase the patient’s drug dosage based on the misinterpreted “low” result). All laboratory estimations, whether communicated verbally, printed report or electronically, must therefore include an appropriate unit of measurement records with the result. |
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Reference |
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Synonym to measurement unit
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Unit, unit of measurement, reference | ||
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Definition of measurement unit
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Real scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared to express the ratio of the two quantities as a number (reference 1) | ||
Scope
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Measurement unit is a generic concept of a quantity of any dimension, including non-SI unit and SI unit | ||
Description |
The specific kind-of-quantity specifies the type of the corresponding measurement unit, e.g. mmol/L is the corresponding measurement to the kind-of-quantity, substance concentration. Note: There can be a many-to-many relation between the kind-of-quantity and the type of meaurement unit. Hence, a kind-of-quantity should be stated in the description of the measurement. Examples The corresponding measurement unit to mass concentration and density can be µg/L or mg/L The corresponding measurement unit to mass fraction or volume fraction can be per mille or one out of thousand (symbol: ‰) |
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Rule |
Reason |
Example |
One expression per unit |
To ensure unambiguity in reporting values, only one expression for a unit of a given magnitude should be used. Various expressions may cause errors in communication between health personnel and organisations with potential patient mistreatments as consequences | Millimole per liter |
Use multiples and submultiples |
To present numerical values in the interval of 0.1–999 and to make values with very large or very small numerical values readable, the units can be combined with SI prefixes, expressed as either SI prefix symbols or SI prefix factors (numerical values) | |
One SI prefix per unit |
To ensure unambiguity in reporting values, only one expression for a unit of a given magnitude should be used. Various expressions may cause errors in communication between health personnel and organisations with potential patient mistreatments as consequences | |
One or SI prefix factors are units for dimensionless kind-of-quantities
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Reference |
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Scope |
A description of hierarchical categories of units that are graded according to preference | ||
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Description |
Preferable, SI units or internationally accepted non-SI units should be applied. However, due to technical reasons this may not always apply. Prefered units according to a following hierarchical category system:
Note: Internationally accepted non-SI units are traceable to SI units. |
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Reference |
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