The present application relates to anti-PD-L1 antibodies and their use to detect PD-L1 in a sample from a subject. In some embodiments, the subject has been treated with a therapeutic anti-PD-L1 antibody and an anti-PD-L1 described herein does not compete for binding to PD-L1 with the therapeutic anti-PD-L1 antibody. In some embodiments, the anti-PD-L1 antibody is linked to a N detectable moiety, such as a fluorophore and the anti-PD-L1 antibody is used to detect PD-L1 in a subject using flow cytometry.