The combined use of the global coagulation tests prothrombin time (PT) and activated partial thromboplastin time (APTT) is usually used to identify RBDs of clinically significant severity, but not FXIII deficiency. 

Specific assays for each different coagulation factor are necessary to determine the level of deficiency (severe, moderate, mild). Immunoassays to measure the conserved antigen levels are not strictly necessary for diagnosis and treatment but are necessary to distinguish type I from type II deficiencies. 1

Most RBDs are expressed phenotypically by a parallel reduction of plasma factors measured by functional and immuno-assays (so-called type I deficiencies). Qualitative defects, characterized by normal, slightly reduced, or increased levels of antigen levels contrasting with much lower or undetectable functional activity (type II), are less frequent  2 - 3