Laboratory testing supports most clinical decision-making; however, the concept of a fixed “normal” value is theoretical.

Studies have suggested that approximately 7 out of 10 clinical decisions rely on laboratory data. Given this significant role, defining appropriate normal values or reference ranges is essential. However, most laboratory “normal” ranges are fixed rather than identified and reflect methodological constraints and, at times, economic or policy influences. A truly exact normal range for laboratory values exists only theoretically.

This article explains how normal values or reference ranges are established and why their interpretation requires clinical expertise. For experienced physicians, laboratory results are not merely numbers that must be memorized. Interpretation requires context, background knowledge, and careful clinical judgment.

How Reference Ranges Are Ideally Derived

In principle, each laboratory should establish a reference range for each analyte. This process required sampling at least 120 healthy individuals per demographic group and defining the central 95% of the measured values as the reference range.

An advantage of this method is instrument-specific calibration, which enhances comparability during longitudinal follow-up. However, by definition, only 5% of healthy individuals fall outside the reference range.

In practice, most laboratories do not generate normal values or reference ranges. Instead, they rely on reference books, manufacturer specifications, or reference ranges reported in the medical literature and adjust them mathematically for instrument-specific differences.

These differences may have been substantial. For example, in 2024, a study demonstrated that troponin I concentrations varied by more than 400%, depending on the assay method and instrumentation.

Although harmonization across assays is clinically advantageous, it is costly and may be limited by commercial considerations. To date, only a limited number of analytes, including cholesterol, creatinine, and hemoglobin A1c, have achieved significant harmonization.

Another caveat is that many reference ranges reported in the medical literature were established in research hospitals, where participants followed standardized diets and daily routines. Blood samples were collected in the morning after waking and fasting. Therefore, for many analytes, the true values in healthy, active individuals may be higher than those indicated in the laboratory reference ranges. Examples include creatine kinase, which tends to be higher in physically active individuals than in those who are ill, and alkaline phosphatase, which can increase postprandially. 

Expert Consensus

Expert panels may adjust statistically derived or reference ranges from the medical literature.

For example, in the US, the average BMI is approximately 30. Defining the central 95% as normal would therefore classify overweight as normal. Instead, expert consensus established a BMI reference range of 18.5-25.

Blood pressure thresholds have also been revised downward over time following expert deliberation and outcome-based evidence. The thyroid-stimulating hormone reference range has narrowed because assay sensitivity has improved. Cholesterol targets have progressively declined in response to cardiovascular outcome data.

Age and Sex Differences

Reference ranges vary according to age and sex. An adult woman with a hemoglobin level of 12.0 g/dL falls within the normal range. A newborn with the same value is classified as anemic. Prostate-specific antigen levels increase with age. Alkaline phosphatase concentrations are higher in growing children.

Sex-specific differences have also been well established. A hemoglobin concentration of 12.0 g/dL is considered normal in adult women but would meet the criteria for anemia in adult men. High-density lipoprotein cholesterol, ferritin, creatinine, and other analytes have sex-specific ranges. Reference ranges for nonbinary and transgender individuals remain under investigation.

Physiologic State and Ethnicity

Physiologic status varies across life stages, including childhood, pregnancy, and aging, and can affect enzyme and hormone concentrations.

Variations in ethnicity have also been reported previously. On average, vitamin D concentrations are lower in Black individuals than in White individuals. Lower levels of vitamin D-binding proteins may partially account for this difference, whereas the bioavailable vitamin D levels appear to be similar.

The incorporation of ethnic differences into reference ranges is complex and socially sensitive.

Methodological Variation

Analytical techniques significantly influence laboratory results.

Arterial blood gas measurements obtained in emergency departments may differ from those processed in central laboratories within the same institution, with differences of 0.5-1.0 g/dL being common.

Blood glucose levels measured using portable glucometers are typically lower than those measured in the laboratory. Vitamin D levels vary depending on whether immunoassay or mass spectrometry is used for their determination.

Economic and Policy Influences

Health economic and policy objectives also shape thresholds considered “normal.”

The recommended daily nutrient intake was originally defined to prevent a deficiency of 98% in optimal intake.

For example, optimal folate concentrations may exceed 13 ng/mL, whereas the recommended thresholds are often set above 7 ng/mL. Serum vitamin D levels illustrate similar ambiguity: Serum concentrations above 20 ng/mL correlate with normal parathyroid hormone levels and are frequently considered sufficient. However, populations with high sun exposure historically exhibit levels between 50 ng/mL and 80 ng/mL, suggesting alternative interpretations of physiologic norms.

Laboratory testing is indispensable in modern medicine; however, its interpretation requires clinical judgment. Medical training often presents normal values or reference ranges as fixed constants that can be memorized. In reality, exact normal values and reference ranges exist only theoretically.

Reference ranges are dynamic and context-dependent numbers shaped by methodology, population characteristics, expert consensus, and policy considerations. They should be interpreted in the context of clinical findings rather than applied theoretically.

This story was translated from Univadis Germany, part of the Medscape Professional Network.