Normal Laboratory Values

Laboratory reference ranges (often referred to as reference values or reference intervals) for blood, urine, cerebrospinal fluid (CSF), stool, and other fluids vary based on several factors, including the demographics of the healthy population from which specimens were obtained and the specific methods and/or instruments used to assay these specimens.

See American Board of Internal Medicine (ABIM) for an extensive list of laboratory test reference range values.

Laboratories that are accredited by the College of American Pathologists (CAP), Clinical Laboratory Improvement Amendments (CLIA), Commission on Office Laboratory Accreditation (COLA), or the Food and Drug Administration (FDA) are required to establish and/or validate their own reference ranges for FDA-approved assays and for laboratory developed tests (LDTs). Thus, any given result should be interpreted based on the reference range of the laboratory in which the test was done. The laboratory typically provides these values with the test result. Reference ranges on other types of body fluids (eg, synovial, peritoneal, pleural, pericardial) have not been widely established. Therefore, these tests may be considered LDTs. Analyte reference ranges from LDTs are established by the individual laboratory performing the testing and typically vary more than FDA-approved reference ranges do.

Reference ranges are determined using a cohort of individuals who are otherwise healthy in the context of an analyte being measured. For example, a reference range for glucose by age-group and sex should be determined in a corresponding population of people who do not have comorbidities related to diabetes. A main goal in determining reference ranges is to identify laboratory test values that distinguish between patients with and without the clinical condition being evaluated.

When determining reference ranges, it is also important to evaluate expected physiological changes in the context of the specified patient population (eg, age, sex, pregnancy, menopause, sample time variation, geographic location). Clinical laboratories consider all of these factors when establishing or validating reference ranges.

While reference ranges do not usually allow 100% distinction between health and disease, they generally represent 95% of the otherwise healthy population. To establish a reference range, it is recommended that laboratories use at least 120 reference individuals from the population. However, when validating reference ranges that have already been established, most laboratories use 40 samples from the reference population. An understanding of how reference ranges are derived and accepted along with inherent imprecision can help guide clinical interpretation and patient management.