Congenital Afibrinogenemia and Hypofibrinogenemia: Genetic Basis, Pathophysiology, Laboratory Diagnosis, and Management

Congenital fibrinogen disorders (CFDs) comprise quantitative deficiencies—afibrinogenemia and hypofibrinogenemia—and qualitative defects—dysfibrinogenemia and hypodysfibrinogenemia. This review is confined to the quantitative disorders, and the term quantitative CFDs is used throughout in preference to the wider umbrella term; qualitative defects are considered only insofar as they must be distinguished at the bench. Afibrinogenemia results from biallelic loss-of-function variants in FGA, FGB, or FGG, and is defined by fibrinogen activity below the analytical limit of detection with undetectable antigen; hypofibrinogenemia results from monoallelic variants and is defined by a proportionate reduction in activity and antigen. Several authoritative reviews of this field have appeared recently. The contribution intended here differs in emphasis. Rather than restate the stepwise diagnostic approach, we set out what actually goes wrong at the bench: the analytical variability among fibrinogen methods, the behavior of the Clauss assay near its limit of detection, and overestimation by prothrombin time-derived fibrinogen. We also address interference from anticoagulants and sample factors, the requirement for laboratory-specific reference intervals, the interpretation of discordant results, and the molecular workflow, including copy-number analysis and the handling of variants of uncertain significance. We also address a question that the descriptive literature largely leaves open: why patients carrying identical variants, sometimes within a single family, differ so markedly in phenotype. We propose a four-layer framework—cis-acting effects at the fibrinogen locus, hepatocyte proteostasis and endoplasmic reticulum quality control, trans-acting genetic modifiers, and acquired factors—and review the evidence supporting each. Population genomic data are reconsidered: current estimates derived from gnomAD suggest that predicted-deleterious fibrinogen genotypes are considerably more frequent than clinically ascertained disease, a discrepancy that reflects incomplete penetrance and the limits of in silico prediction rather than a hidden burden of undiagnosed severe disease. Management recommendations are presented with the strength of the supporting evidence made explicit. Fibrinogen concentrate is a first-line replacement. Widely accepted targets are a peak > 1.5 g/L before major surgery, and >1.0 g/L before minor procedures; >1.0 g/L postoperatively until hemostasis, and >0.5 g/L until wound healing; and a trough ≥ 1.0 g/L in pregnancy, rising to ≥1.5 g/L peripartum. All data are derived from expert consensus, registry data, and small interventional series rather than randomized trials, and are presented here as starting points for individualized care. Paradoxical thrombosis, its uncertain mechanism, and the possibility that replacement precipitates it are treated as an unresolved problem rather than a footnote.