Methods for determining the presence of heparin/platelet factor 4 antibodies in a sample suspected to contain heparin/platelet factor 4 antibodies are provided, along with apparatus suitable for performing the methods. The method depends upon a color visualization indicating the presence or absence of heparin/platelet factor 4 antibodies in the sample. Preferred methods comprise contacting the sample with particles being complexed to platelet factor 4 (PF4) and which particle-complexed PF4 reacts specifically with heparin/platelet factor 4 antibodies, passing the sample/particle mixture through a filter, and then analyzing the color of the filtrate. The presence of heparin/platelet factor 4 antibodies in the sample is established where the color of the filtrate is substantially different from the color of the receptor-bearing particles.