Provided herein are methods for detecting an antigen or for detecting expression of a chimeric antigen receptor (CAR). The methods include obtaining a sample from a subject, contacting the sample with a fusion protein comprising a reporter fused to a single chain antibody specific to the antigen or fused to an extracellular domain of an antigen targeted by the CAR or fused to Protein L and assaying the activity of the reporter, wherein presence of reporter activity or increase in reporter activity relative to a reference value is indicative of presence of the antigen or presence of the expression of the chimeric antigen receptor in the sample.