Calibrator/control for simultaneous assay of proteins capable of complexing with one another

A method of calibrating or running controls in an assay for sFlt-1 and PlGF, the method comprising:

• selecting as a calibrator or control a composition comprising known amounts of non-immobilized sFlt-1 and non-immobilized PlGF, wherein one or more amino acids or one or more non-amino acid groups of the non-immobilized sFlt-1 and/or the non-immobilized PlGF have been deleted, modified, or replaced with a different amino acid or non-amino acid group or groups thereby reducing or substantially eliminating formation of a non-covalent association complex between the non-immobilized sFlt-1 and the non-immobilized PlGF;
• contacting the composition with a first immobilized receptor specific for sFlt-1 and a second immobilized receptor specific for PlGF, thereby forming a first complex of the non-immobilized sFlt-1 and the first immobilized receptor, and a second complex of the non-immobilized PlGF and the second immobilized receptor;
• and
• detecting amounts of the first complex and the second complex, wherein amounts of the first complex and the second complex are compared to the known amounts of the non-immobilized sFlt-1 and the non-immobilized PlGF to calibrate or control the assay.