I tested positive for chlamydia. I immediately started taking Doxy (the 7-day antibiotic). A couple days into that treatment, I had vaginal sex with a different male partner. We used a condom but it kept moving around and I’m not sure it was correctly used. 

I told the first male partner to get tested and his came back negative. I also told the second male partner but he blocked my number so I can’t contact him to follow up. 

Once I took all 7 days of the antibiotics, the following day I went to LabCorp to get a urine test for chlamydia. I just got the results back that I am still positive. 

For reference, the timeline is: I started the antibiotics on September 12. Which means I took my last pill on the 19th. Two days later (September 21), I went to get re-tested at LabCorp. I received my results on September 24. 

Questions: 

Does the unprotected sex that occurred while I was taking the antibiotics make the treatment essentially ineffective?

Is the positive test result reliable, or is it too soon after my antibiotic treatment has ended to receive a negative result? If so, how long should I wait to get re-tested?

I'm not surprised that your test remained positive on day 8 following initiation of doxycycline treatment.  Doxy is the preferred and recommended treatment for chlamydia with cure rates of more than 97%.  The tests for chlamydia however can remain positive after treatment due to detection of dead chlamydia for up to two weeks after successful treatment.  Presuming that your symptoms have improved, I would wait a week more before repeating the test.  At that time I anticipate the test will be negative.

As for your partners, your initial partner may have had a falsely negative test (if he did test) or possibly you had chlamydia already at the time you had sex with him and it was not transmitted.  Your 2nd partner should be fine. Condoms work well as long as they are worn throughout the encounter and the head of the penis is covered by the condom- slippage typically does not reduce the protective effect of condoms.  

I hope this information is helpful.  EWH