External Quality Assessment Failures in Small Laboratories: Root Causes Across Five Survey Cycles

Abstract

Small laboratories -- defined here as those processing fewer than 200 samples per analyte per month -- are reported anecdotally
to fail external quality assessment (EQA) surveys more often than larger laboratories, but the specific root causes behind these
failures are not well characterised separately from the failure rate itself. We reviewed root cause investigation records for every
EQA failure reported by 34 small laboratories across five consecutive survey cycles, categorising each investigated failure by its
identified root cause. Calibration or reagent lot-related causes accounted for the largest category (38.4%), followed by
insufficient sample volume for low-throughput analytes (24.1%) and infrequent operator exposure to a seldom-run assay (19.7%).
We argue that these three dominant categories share a common structural driver -- insufficient testing volume to detect drift or
maintain competence between EQA cycles -- and propose targeted mitigations for small laboratories that differ from the generic
EQA remediation guidance written primarily for higher-volume settings.