This guidance was prepared on behalf of the International Council for Standardisation in Haematology (ICSH) by an international working group of clinicians and scientists. The document focuses on tests and assays used for the assessment of fibrinogen function, particularly in the scenario of bleeding disorders. Thrombin clotting time (TT) is used as a screening test in some laboratories and also has some utility when direct anticoagulants are in use. The Clauss fibrinogen assay remains the method of choice for the assessment of fibrinogen function, but there are some situations where the results may be misleading. Prothrombin time derived fibrinogen assays are frequently used, but should be interpreted with caution; the results are not interchangeable between different methods and fibrinogen can be overestimated in certain clinical scenarios. Viscoelastic point of care methods may be helpful in emergency situations, while Reptilase time (and similar tests) are useful combined with TT in distinguishing heparin contamination of samples (i.e., if an incorrect blood draw is suspected) and the presence of direct thrombin inhibitors. Fibrinogen antigen assays should be used in the investigation of functional fibrinogen abnormalities; fibrinogen antigen and genetic testing are recommended in the confirmation of congenital fibrinogen disorders. The following recommendations for fibrinogen function assessment are based on published literature and expert opinion and should supplement local regulations and standards.