Introduction

If you live in Greece and have received blood test results, you've probably noticed the indicated reference interval — the range of "normal" values for healthy people. But it's important to understand: the upper or lower boundary of this interval is not always the threshold at which doctors make clinical decisions. We'll explain what a clinical decision threshold (cut‑off) is and how it differs from the familiar concept of "normal."

What is a reference interval (RI)?

This is a statistical range that covers approximately 95% of values in healthy people — a conditional population.

Values outside this range may require additional monitoring, but don't necessarily indicate disease — since approximately 5% of healthy people can have results outside the normal range (Testing.com).

What is a clinical decision threshold (cut‑off)?

This is a specific analyte value at which medical intervention or further examination is prescribed — associated with a specific pathology or group of diseases (PubMed).

For example, HbA1c levels above a certain threshold are used to diagnose diabetes, even if they're close to the upper limit of normal (pmc.ncbi.nlm.nih.gov).

How does a threshold differ from a reference interval?

Parameter Reference Interval Clinical Decision Threshold (cut‑off)
Target Group Healthy people People with specific diseases
Comment Allows determination of whether a result deviates from normal Divides patients into likely sick and healthy
Basis Statistics of healthy population (~95%) Clinical study data (ROC curves)
Result outside Possible reason for monitoring Probable diagnosis or doctor's action

Results outside the reference interval require careful attention, but don't always mean disease. Cut‑off indicates exactly a higher risk of having a specific condition (Testing.com, mediasphera.ru, meganorm.ru, "Children's Doctor" Clinic).

In simple terms

  • Reference interval — is a guide: "for most people, the usual value."
  • Clinical decision threshold — is the boundary after which doctors consider the presence of disease with high probability.
  • The threshold can be inside, outside, or independent of the reference range — it all depends on diagnostic goals (sensitivity, specificity, predictive value).

Why is this important?

  • Sometimes patients see that a result is "above normal" and immediately worry — but this isn't always a reason for fear.
  • The doctor uses cut‑off thresholds to make a decision: whether to prescribe treatment, additional examination, or monitoring.
  • Especially important in neurology: different biomarker tests can have clear thresholds for confirming or excluding diseases.

Advice for patients

  • Always pay attention to the established threshold for a specific test, not just the reference interval.
  • If a result is outside normal — discuss with your doctor whether additional diagnostics are needed.
  • Remember that for some groups of patients, cut‑off thresholds may differ (age, concomitant diseases, etc.).
  • It's always important to consider the result in combination with the clinical picture, not based on one indicator. Such practice is widely used in international guidelines, including IFCC and other medical organizations.

Links to European and Greek popular science resources

Links to sources from Greece and Europe are provided:

  • IFCC / C‑RIDL — international review of differences between reference intervals (RI) and clinical decision limits (CDLs) (whitehatcom.com, researchgate.net)
  • Review of Distinguishing reference intervals and clinical decision limits — international review of RIs vs CDLs (tandfonline.com)
  • EUCAST materials (European Committee on Clinical Sensitivity Limits) — examples of cut‑off in the context of microbiology, sensitivity classification (eody.gov.gr, pergamos.lib.uoa.gr)

Conclusion

The difference between reference intervals and clinical decision thresholds is important for everyone who receives laboratory tests to understand. Knowing that cut‑off specifically indicates the probability of diagnosing a specific disease helps avoid unnecessary anxiety and correctly perceive medical information. For any doubts, it's better to discuss the results with a neurologist.