Pass/Fail Guidelines
Pass/Fail Determination
The verification outcome is determined by counting how many reference individual results fall outside the proposed reference interval limits:
Number Outside
Count of results where Value < Lower Limit or Value > Upper Limit
The total number of reference individual results that fall outside the proposed reference interval. This count drives the pass/fail decision.
Percent Outside
(Number Outside / Total Samples) × 100
The percentage of results outside the interval. The CLSI EP28 threshold is 10% — equivalent to ≤ 2 out of 20 samples for the minimum study size.
Pass/Fail Rule (CLSI EP28):
- ≤ 10% outside (≤ 2/20): Verification passes — the preestablished reference interval is confirmed for your laboratory.
- 3–4 out of 20 outside (15–20%): Borderline result — CLSI recommends obtaining an additional 20 reference individuals and repeating the study. If ≤ 2 of the new 20 fall outside, the interval is verified. If 3 or more of the new set fall outside, or if 5 or more of any single dataset are outside, the interval is not verified.
- ≥ 5 out of 20 outside (≥ 25%): Verification fails — the preestablished interval cannot be confirmed for this laboratory's population or method.
When Verification Fails: A failed verification indicates that the proposed reference interval may not be appropriate for your patient population or test method. Investigate potential causes — population demographic differences, preexamination variability, or analytical performance issues. If the interval cannot be verified, consider establishing a laboratory-specific reference interval according to CLSI C28-A3c (≥ 120 reference individuals). Consult your laboratory director before releasing patient results.