The invention refers to an in vitro method of functionally determining at physiological conditions deficiencies in the lectin pathway of the complement system, the method comprising the steps of (a) providing a sample of mammalian blood, serum, plasma or another body fluid; (b) preventing in the sample the activation of the classical pathway by contacting the sample with an inhibitor of a molecule of the C1 complex of the complement system; (e) preventing in the sample the activation of the alternative pathway; (d) activating the lectin pathway in the sample; and (e) determining in the sample any activation of the autologous C5b-9 complex. The invention also refers to a kit for functionally determining in a body fluid from a mammal deficiencies in the lectin pathway of the complement system, which kit comprises the separate items (a) an inert carrier and a substance activating the lectin pathway; (b) a diluent comprising an inhibitor of a molecule of the C1 complex; and an antibody against the autologous C5b-9 complex.