In medicine, a biological reference interval (BRI) is the range of values for a particular analyte (a substance measured in the blood or other body fluids) that is considered to be normal in a healthy population. The BRI is used by clinicians to interpret laboratory test results and to identify potential medical problems.

The BRI is determined by studying a large group of healthy people and calculating the 2.5th and 97.5th percentiles of the values for the analyte. The 2.5th percentile is the value below which 2.5% of the healthy population falls, and the 97.5th percentile is the value above which 97.5% of the healthy population falls. Any value that falls outside of these two limits is considered to be abnormal.

It is important to note that the BRI is not a fixed value. It can vary depending on a number of factors, including age, sex, race, ethnicity, and lifestyle factors such as diet, exercise, and smoking. For this reason, it is important to consult with a healthcare professional to interpret laboratory test results.

Here are some examples of biological reference intervals for common laboratory tests:

* Blood glucose: 70-100 mg/dL

* Cholesterol: 125-200 mg/dL

* Triglycerides: 100-150 mg/dL

* Creatinine: 0.6-1.2 mg/dL

* Hemoglobin: 13-18 g/dL (women) or 14-18 g/dL (men)

* Hematocrit: 37-47% (women) or 40-50% (men)

The BRI can be a helpful tool for clinicians in diagnosing and managing diseases. However, it is important to remember that the BRI is just a guide. It is not a substitute for clinical judgment.