Determination of Fibrinogen Ratio Cutoff Limits Using Indirect Reference Interval Methodology

Discordant fibrinogen antigen to activity ratios are utilized by clinicians as evidence of dysfibrinogenemia. Abnormal ratio cutoffs implemented by clinical laboratories are typically determined by validation studies that include limited numbers of samples. We here utilize large datasets of stored clinical results and apply indirect reference interval (RI) methodology to determine fibrinogen ratio cutoffs suggestive of dysfibrinogenemia.

Panel results comprised of fibrinogen antigen and activity measurements and calculated ratios of antigen to activity were retrieved for analysis. Two datasets were analyzed: (1) 3693 unique patient results in which antigen concentrations were measured by radial immunodiffusion (RID) from January 2019 to April 2023, and (2) 2192 patient results with antigen concentrations measured using turbidimetry immunoassay between April 2024 and March 2025. Both datasets were analyzed using the RefineR algorithm to estimate the corresponding RI of fibrinogen activity and antigen, and ratio cutoffs.

Estimated fibrinogen antigen/activity ratio cutoffs were within close range (< 8% difference) to the validated cutoffs used by our laboratory: 1.17 versus 1.23 and 1.06 versus 1.01 using antigen RID and turbidimetry assays, respectively. Contrarily, estimated upper RI limits of antigen and activity were higher by 38.4%-57.2% than validated limits, and RI was wider by 41.8%-76%.

The RefineR algorithm could be used to determine fibrinogen ratio cutoffs with the advantage of including significantly larger numbers of available clinical results. For antigen and activity, the algorithm could not separate out acute-phase elevated fibrinogen (activity and antigen) results and overestimated the upper RI limits relative to the clinically validated cutoffs.