The present invention provides an immunodiagnostic method for determining procalcitonin and procalcitonin derivatives in a biological sample of a patient for diagnostic purposes, in particular in the monitoring and control of treatment and the monitoring of the progression of a local or systemic bacterial infection, inflammation, sepsis or neurodegenerative disease. In particular, the method detects molecular forms of procalcitonin, or procalcitonin partial peptides derived therefrom, having the amino acids alanine and proline (Ala-Pro, AP) in positions 1 and 2 of the amino terminus of the complete procalcitonin 1-116 (SEQ ID NO: 1.) Also disclosed are antibodies and kits for carrying out such a method.