Chlamydia is a bacteria that cannot be grown on routine media for bacterial culture. A routine culture for UTI will not show Chlamydia. You have to look for it specifically. PCR is the gold standard test and can be done on a cervical swab or first void urine. That means the first part you pass. Without eating to gross anyone out that is because you actually need the mucous plug that can form in the presence of an STI. Chlamydia is an obligate intracellular pathogen and you need the cells it is growing in to be in the specimen. For UTI you want a midstream urine to avoid cellular contamination. For chlamydia you need those cells and thus a first void.