How I Investigate Borderline Prolonged aPTT: An Integrated Laboratory Approach to Mixing Studies and Factor Analysis

Borderline prolongation of routine clotting assays-particularly the activated partial thromboplastin time (aPTT)-is a common interpretative challenge in laboratory hematology. These reproducible but mildly prolonged results often lie just beyond the upper reference limit and can trigger unnecessary follow-up, delays, or misinterpretation. Conventional tools such as 1:1 mixing studies may appear corrective due to dilutional masking of weak inhibitors, residual anticoagulants, or low-grade lupus anticoagulant (LA) activity, especially in assays with limited sensitivity. In our laboratory, borderline prolongations prompt a structured, hypothesis-driven workflow that escalates interpretative resolution in sequential steps. We begin with verification of analytical validity and exclusion of confounders (e.g., DOACs, pre-analytical errors), then apply conventional mixing studies interpreted via quantitative correction indices (e.g., Rosner, percent correction). When results remain indeterminate or discordant, we escalate to enhanced mixing ratios (e.g., 4:1, 1:4), followed by factor assays and dilutional analysis including parallelism assessment. Here, we outline an integrated decision-support framework for investigating borderline aPTT prolongation. The framework aligns traditional and enhanced mixing strategies with factor-level interpretation into a coherent, stepwise pathway featuring explicit stop-go points. It is intended for use in advanced or referral coagulation laboratories and emphasizes interpretative rigor over binary thresholds. By optimizing existing tools within a layered interpretative structure, this approach seeks to reduce false reassurance and enhance clarity in borderline aPTT interpretation.