Troubleshooting / Common Issues

Study fails — % Out of Limits exceeds 10% for PT or INR

Possible causes

–Reference interval limits are too narrow for the local population
–Normal subject pool includes individuals with subclinical coagulation abnormalities
–Incorrect ISI value entered — check the reagent lot's package insert
–Reagent lot variability — some lots have wider normal distributions than others

Recommended action

First verify the ISI value and subject eligibility criteria. Then compare the Proposed Limits against the observed 2SD Range — if the limits are well inside the 2SD Range, widen them. Consult the reagent manufacturer's package insert for their recommended PT and INR reference interval.

INR Geometric Mean is not close to 1.0

Possible causes

–ISI value entered incorrectly (wrong lot or transposed digits)
–NPM is being computed from a non-normal population (subjects with elevated PT)
–Instrument-reagent combination has shifted since the ISI was assigned by the manufacturer

Recommended action

Verify the ISI from the package insert. If the ISI is correct and the NPM appears reasonable, the reagent lot may have a calibration offset relative to the manufacturer's reference instrument. Contact the manufacturer if INR geometric mean deviates substantially from 1.0 (e.g., >1.1 or <0.9) with a correct ISI.

Analysis not available — fewer than 20 samples

Possible causes

–Insufficient number of normal subjects enrolled in the study
–Some PT values were excluded due to invalid entries (zero or non-numeric values)

Recommended action

At minimum 20 valid PT values (all > 0) are required to run the analysis. Add additional normal subjects to meet the minimum. For reference interval establishment, ≥60–120 subjects are recommended for statistical reliability.

PT and INR pass/fail status disagree

Possible causes

–ISI is far from 1.0 — a wide ISI amplifies differences between PT and INR distributions
–INR reference limits are too narrow relative to the derived PT limits
–When Compute INR from PT is disabled, manually entered INR limits may not be consistent with the PT limits

Recommended action

When Compute INR from PT is enabled, the INR limits are derived directly from the PT limits, so pass/fail status should typically agree. If they disagree when the checkbox is enabled, verify the ISI — an extreme ISI value can cause the INR distribution to be disproportionately spread relative to the PT distribution.