The present invention relates to a method of in vitro determining functional deficiencies in the lectin pathway of the complement system, the method comprises the steps of (a) diluting a mammalian sample of body fluid with a diluent comprising one or more inhibitors of the activation of the classical and the alternative pathways of the complement system; (b) activating the lectin pathway of the complement system in the sample obtained from (a); and (c) determining in the sample obtained from (b) the activation of one or more of the complement factors C3, C4, or one or more of the components of the C5-C9 complex. The invention furthermore relates to kits for use in connection with the above-mentioned method, the first kit comprises i) a first component comprising a carrier, one or more inhibitors of the classical and the alternative complement pathways and a diluent; and ii) a second component comprising one or more substances for activation of the lectin complement pathway and optionally an inert carrier. The second kit comprises a container comprising a predetermined amount of one or more inhibitors of the classical and the alternative complement pathways and a diluent, wherein the container is adapted for receiving a predetermined amount of sample, so that when the predetermined amount of sample is added, the concentration of the one or more inhibitors is an inhibitory effective concentration of the classical and alternative pathways, but not the lectin pathway.