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Reference ranges (reference intervals) for blood tests are sets of values used by a health professional to interpret a set of medical test results from blood samples. Reference ranges for blood tests are studied within the field of clinical chemistry (also known as "clinical biochemistry", "chemical pathology" or "pure blood chemistry"), the area of pathology that is generally concerned with analysis of bodily fluids.
Blood test results should always be interpreted using the reference range provided by the laboratory that performed the test.[1]
Interpretation
A reference range is usually defined as the set of values 95 percent of the normal population falls within (that is, 95% prediction interval).[2] It is determined by collecting data from vast numbers of laboratory tests.
Plasma or whole blood
In this article, all values (except the ones listed below) denote blood plasma concentration, which is approximately 60–100% larger than the actual blood concentration if the amount inside red blood cells (RBCs) is negligible. The precise factor depends on hematocrit as well as amount inside RBCs. Exceptions are mainly those values that denote total blood concentration, and in this article they are:[3]
- All values in Hematology – red blood cells (except hemoglobin in plasma)
- All values in Hematology – white blood cells
- Platelet count (Plt)
A few values are for inside red blood cells only:
- Vitamin B9 (folic acid/folate) in red blood cells
- Mean corpuscular hemoglobin concentration (MCHC)
Units
- Mass concentration (g/dL or g/L) is the most common measurement unit in the United States. Is usually given with dL (decilitres) as the denominator in the United States, and usually with L (litres) in, for example, Sweden.
- Molar concentration (mol/L) is used to a higher degree in most of the rest of the world, including the United Kingdom and other parts of Europe and Australia and New Zealand.[4]
- International units (IU) are based on measured biological activity or effect, or for some substances, a specified equivalent mass.
- Enzyme activity (kat) is commonly used for e.g. liver function tests like AST, ALT, LD and γ-GT in Sweden.[5]
- Percentages and time-dependent units (mol/s) are used for calculated derived parameters, e.g. for beta cell function in homeostasis model assessment or thyroid's secretory capacity.
Arterial or venous
If not otherwise specified, a reference range for a blood test is generally the venous range, as the standard process of obtaining a sample is by venipuncture. An exception is for acid–base and blood gases, which are generally given for arterial blood.
Still, the blood values are approximately equal between the arterial and venous sides for most substances, with the exception of acid–base, blood gases and drugs (used in therapeutic drug monitoring (TDM) assays).[6] Arterial levels for drugs are generally higher than venous levels because of extraction while passing through tissues.[6]
Usual or optimal
Reference ranges are usually given as what are the usual (or normal) values found in the population, more specifically the prediction interval that 95% of the population fall into. This may also be called standard range. In contrast, optimal (health) range or therapeutic target is a reference range or limit that is based on concentrations or levels that are associated with optimal health or minimal risk of related complications and diseases. For most substances presented, the optimal levels are the ones normally found in the population as well. More specifically, optimal levels are generally close to a central tendency of the values found in the population. However, usual and optimal levels may differ substantially, most notably among vitamins and blood lipids, so these tables give limits on both standard and optimal (or target) ranges. In addition, some values, including troponin I and brain natriuretic peptide, are given as the estimated appropriate cutoffs to distinguish healthy people from people with specific conditions, which here are myocardial infarction and congestive heart failure, respectively, for the aforementioned substances.[7][8][9]
Variability
References range may vary with age, sex, race, pregnancy,[10] diet, use of prescribed or herbal drugs and stress. Reference ranges often depend on the analytical method used, for reasons such as inaccuracy, lack of standardisation, lack of certified reference material and differing antibody reactivity.[11] Also, reference ranges may be inaccurate when the reference groups used to establish the ranges are small.[12]