Low complement factor C4 is usually considered a valuable screening tool for patients with the potentially life-threatening hereditary angioedema with C1-inhibitor (C1-INH) deficiency (C1-INH-HAE). However, there are patients with C1-INH-HAE presenting with normal C4 levels. This means, that C1-INH-HAE may potentially be overlooked, if screening is performed only by measurement of C4. It has been suggested that measurement of C4 activation products is better suited to avoid false negative results. Our aim was to investigate whether total antigenic C4 or non-functional C4c is a better measure of the increased C4 activation in C1-INH-HAE patients.Two different monoclonal antibodies (mAb) to human C4 were produced: one had specificity for the β-chain of C4 and would thus react with both functional and non-functional C4, and the other was developed against the factor I cleavage site on the α3-domain of C4 and was thus specific for activated, non-functional C4c. With these mAb we investigated plasma from 19 Danish C1-INH-HAE patients in three different enzyme-linked immunosorbent assays (ELISAs): a total antigenic C4 assay, a functional C4 assay and an assay measuring non-functional C4c.The amount of total antigenic C4 varied considerably between patients and 2 patients had total antigenic C4 levels in the normal area. Functional C4 was low in all C1-INH-HAE patients. A C4c/C4 ratio showed that around half the C4 measured in patients was non-functional and captured all C1-INH-HAE patients.This study shows that the C4c/C4 ratio seems to be a better diagnostic measure than total antigenic C4 alone. Our findings underline that screening with total antigenic C4 implies a risk of overlooking C1-INH-HAE patients.