A method for measuring a percent of a neutralizing form of an autoantibody to an anti-TNFα drug in a sample, the method comprising: (a) contacting the sample with an acid to dissociate preformed complexes of the autoantibody and the anti-TNFα drug; (b) contacting the sample with a labeled anti-TNFα drug and a labeled TNFα following dissociation of the preformed complexes; (c) neutralizing the acid in the sample to form: (i) a first labeled complex of the labeled anti-TNFα drug and the autoantibody; and/or (ii) a second labeled complex of the labeled anti-TNFα drug, the labeled TNFα, and the autoantibody; (d) subjecting the first labeled complex and/or the second labeled complex to size exclusion chromatography to separate them from free labeled TNFα, free labeled anti-TNFα drug, and/or a complex of labeled anti-TNFα drug and labeled TNFα; (e) measuring an area under the curve of free labeled TNFα in the sample (Free labeled TNFαAUC of the sample) after size exclusion chromatography; and (f) measuring the percent of a neutralizing form of the autoantibody by comparing the AUC of free labeled TNFα measured in step (e c) to an AUC of free labeled TNFα in a first control sample (Free labeled TNFαAUC of the first control sample) and an AUC of free labeled TNFα in a second control sample (Free labeled TNFαAUC of the second control sample) using the equation: Free labeled TNFαAUC of the sample − Free labeled TNFαAUC of the first control sample / Free labeled TNFαAUC of the second control sample * 100, wherein the first control sample is a reference sample containing labeled TNFα and labeled anti-TNFα drug, and the second control sample is a reference sample containing labeled TNFα. (a) contacting the sample with an acid to dissociate preformed complexes of the autoantibody and the anti-TNFα drug; (b) contacting the sample with a labeled anti-TNFα drug and a labeled TNFα following dissociation of the preformed complexes; (c) neutralizing the acid in the sample to form: (i) a first labeled complex of the labeled anti-TNFα drug and the autoantibody; and/or (ii) a second labeled complex of the labeled anti-TNFα drug, the labeled TNFα, and the autoantibody; (i) a first labeled complex of the labeled anti-TNFα drug and the autoantibody; and/or (ii) a second labeled complex of the labeled anti-TNFα drug, the labeled TNFα, and the autoantibody; (d) subjecting the first labeled complex and/or the second labeled complex to size exclusion chromatography to separate them from free labeled TNFα, free labeled anti-TNFα drug, and/or a complex of labeled anti-TNFα drug and labeled TNFα; (e) measuring an area under the curve of free labeled TNFα in the sample (Free labeled TNFαAUC of the sample) after size exclusion chromatography; and (f) measuring the percent of a neutralizing form of the autoantibody by comparing the AUC of free labeled TNFα measured in step (e c) to an AUC of free labeled TNFα in a first control sample (Free labeled TNFαAUC of the first control sample) and an AUC of free labeled TNFα in a second control sample (Free labeled TNFαAUC of the second control sample) using the equation: Free labeled TNFαAUC of the sample − Free labeled TNFαAUC of the first control sample / Free labeled TNFαAUC of the second control sample * 100, wherein the first control sample is a reference sample containing labeled TNFα and labeled anti-TNFα drug, and the second control sample is a reference sample containing labeled TNFα.