Abstract: A method of diagnosing infection in a patient undergoing a treatment of a predetermined type. This method comprises establishing a baseline level of at least one antimicrobial peptide in a specific patient or in a patient population undergoing treatment of a predetermined type, wherein the treatment involves accessing fluid from the patient's peritoneal cavity prior to the patient presenting with a suspected infection; measuring the level of the at least one antimicrobial peptide in a sample taken from the patient after the patient presents with a suspected infection; comparing the baseline level of the at least one antimicrobial peptide to the measured level of the at least one antimicrobial peptide; and diagnosing the patient with an infection if the baseline level of the at least one antimicrobial peptide exceeds the measured level of the at least one microbial peptide by a predetermined amount.