An Automatic Image Based Single Dilution Method for End Point Titre Quantitation of Antinuclear Antibodies Tests Using HEp-2 Cells
Indirect Immunofluorescence (IIF) on Human epithelial (HEp-2) cells test has been the golden standard for identifying the presence of Anti-Nuclear Antibodies (ANA) due to its high sensitivity and the large range of antigens that can be detected. Furthermore, IIF ANA test allows the positive sample strength (sample end point titre) to be reported. Despite its advantages, the IIF ANA test needs to be performed manually, and therefore it is perceived as an expensive and laborious process. This also applies to determining the strength of positive samples (end point titre) which traditionally is done by serially diluting the specimen. In this paper, we present an image-based method which is able to automatically determine the end point titre of positive samples based only on a single screening dilution. This can be done by simulating the manual titration process using a mathematical model of the exposure-density curve. Technically, a new Image Titration Endpoint (ITE) unit based on the model is introduced. Each specimen image is then measured in terms of this unit. Finally, the end point titre for the specimen is determined through a standard curve which specifies the end point titre given an ITE unit. This process is fully automated which would give an advantage over the current digital titration methods. The overall endpoint titre agreement between the proposed approach and the manual serial dilution method in the evaluation of 134 positive samples was 100%. This high agreement demonstrates that the proposed approach is suitable for routine ANA IIF testing in the clinical settings.