The present invention provides an immunodiagnostic method for determining procalcitonin and procalcitonin derivatives in a biological sample of a patient, 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 intact procalcitonin 1-116, or procalcitonin partial peptides derived therefrom that retain amino acids alanine and proline (Ala-Pro, AP) in positions 1 and 2 of the amino terminus of the complete procalcitonin 1-116. The method uses antibodies that selectively bind an epitope comprising amino acids 1 and 2 of procalcitonin 1-116 (SEQ ID NO: 1) and can distinguish between intact procalcitonin 1-116 and for example, procalcitonin 3-116.