Neutropenia as an isolated clinical finding may include aetiologies ranging from severe disease to a transient condition, and differential diagnosis may be challenging. Previous data and clinical experience suggest that low levels of the neutrophil-derived protein human 18 kDa cathelicidin antimicrobial protein (hCAP-18) in the blood are predictive of more severe forms of neutropenia. The objective of this study was to present the results from a newly developed ELISA method that has been used in clinical routine in Sweden since 2018 for quantification of hCAP-18 in blood plasma. Using this method, we report that patients with severe disease analysed during the study period presented with low or undetectable levels of blood plasma hCAP-18, validating its use as screening tool for severe neutropenia. Furthermore, neutropenic patients as a group displayed lower levels of hCAP-18 as compared to blood donors. Within the group of neutropenic patients, those with neutrophil antibodies displayed significantly higher hCAP-18 levels compared to patients with idiopathic neutropenia. By including an analysis of hCAP-18 in the primary investigation of neutropenia, an increased accuracy in differential diagnosis is achieved, thus contributing to reduced costs of neutropenia management.