What is the reference range?

Reference Ranges in Clinical Laboratory Testing

Reference ranges vary depending on the specific laboratory test, assay method, and population characteristics, and should be established according to standardized guidelines for each laboratory parameter to ensure accurate clinical interpretation.

Understanding Reference Ranges

Reference ranges (also called reference intervals) represent the expected values found in healthy individuals and serve as the foundation for interpreting laboratory test results in clinical practice.

Key characteristics of reference ranges include:

• Reference ranges typically include 95% of test results from a presumably healthy population, meaning 5% of healthy individuals may have values outside the range for a single test 1
• For multiple tests (such as complete blood count and metabolic panel), the probability of at least one abnormal result in a healthy person rises to 64% 1
• Reference ranges should be established according to Clinical and Laboratory Standards Institute (CLSI) guidelines using at least 120 healthy volunteers 1

Factors Affecting Reference Ranges

Several factors influence reference ranges and must be considered when interpreting laboratory results:

• Demographic variations: Reference ranges should account for normal variations due to race, ethnicity, age, sex, and gender identity 1
• Assay methodology: Different laboratory methods and platforms can yield different reference ranges for the same analyte 1
• Units of measurement: Units may vary between laboratories (e.g., mol/L vs. pg/mL for natriuretic peptides), making direct comparison difficult 1
• Clinical context: Pre-analytical factors like posture, tourniquet application time, fasting status, time of day, psychological stress, hydration status, and recent exercise can all affect test results 1

Establishing Reference Ranges

When establishing reference ranges, laboratories should follow these practices:

• Use standardized protocols and quality control measures to ensure consistency 1
• Include sufficient sample sizes (ideally 120+ healthy individuals per CLSI guidelines) 1
• Consider verification of manufacturer's or previously established reference ranges if collecting 120 samples is not feasible (using at least 20 healthy subjects) 1
• Participate in external quality assessment programs to improve testing quality 1

Examples of Reference Range Variability

Different laboratory tests have specific considerations for their reference ranges:

• Natriuretic peptides (BNP/NT-proBNP): Reference ranges vary by assay method, with research assays often reporting values 15-20% below commercial assays 1
• Cardiac troponin: The 99th percentile upper reference limit is recommended as the threshold for myocardial injury, but this threshold varies based on how rigorously "normal" populations are defined 1
• Glycated albumin: Reference range in Chinese population established as 10.8% to 17.1% in one multicenter study 1 and 11.89% to 16.87% in another study 1
• Blood lead levels: Reference value ≥5 μg/dL (≥50 ppb) is considered elevated in children 1

Recommendations for Clinical Practice

When using laboratory reference ranges in clinical practice:

• Always consider the specific laboratory's reference ranges rather than general published values 1
• Recognize that laboratory tests should only be used as exclusionary criteria when scientifically justified 1
• For monitoring individual patients, consider using individualized reference ranges based on the patient's own baseline values 1
• Be aware that reference ranges may need to be adjusted for specific populations (e.g., athletes may have different "normal" values than the general population) 1
• When interpreting results near the edges of reference ranges, consider the clinical context and pre-analytical variables 1

Limitations and Pitfalls

Important limitations to consider when using reference ranges:

• Laboratory error can affect interpretation (e.g., for blood lead testing, an allowable error of ±4 μg/dL can result in misclassification of patients) 1
• Reference ranges established in one population may not be applicable to other populations with different characteristics 1
• Some biomarkers lack internationally standardized reference ranges (e.g., glycated albumin) 1
• Carrying forward restrictive laboratory criteria from early clinical trials without adjustment based on experience can unnecessarily limit patient access to treatments 1

By understanding these principles of reference ranges, clinicians can more accurately interpret laboratory results and make appropriate clinical decisions.