A determination method for determining, in an immunodepressed subject, a humoral response due to the presence of a target infectious agent, by detection, in a biological sample of the subject, of at least one target antibody that is susceptible of being produced by the subject when the latter is infected with or has been infected with the target infectious agent. Also, a determination method for determining a humoral response due to the presence of a target infectious agent from a biological sample from an immunodepressed or non-immunodepressed subject, in which the dilution ratio of the sample is selected as a function of the immunodepressed condition or otherwise of the subject concerned.