Blood testing for chlamydia
Blood antibody tests for Chlamydia trachomatis can be done, but they are far less reliable than amplification:


  According to our results, serologic testing for Chlamydia can exclude active infection of the lower genital tract with a high reliability (> or = 95%). However, detection of C. trachomatis can only be reliably achieved by nucleic acid amplification assays.


Low correlation of serology with detection of Chlamydia trachomatis by ligase chain reaction and antigen EIA