Methods for determining the presence or amount of a complex comprising a first and second molecular entity are provided, preferably an sFlt-1:PlGF complex. A determination of the presence or amount of the complex can be used in methods for predicting, detecting, monitoring a disease, or guiding therapy in respect to a disease such as vascular, vascular-related disease, cardiac, cardiac-related disease, cancer, cancer-related disease, preeclampsia, and preeclampsia-related disease. Determining sFlt-1:angiogenic factor complex is particularly useful for predicting and detecting preeclampsia in early stages of gestation and in stages of the disease where clinical evaluation may be uninformative.