Chlamydia Infection Timeline: What Does a Delayed Positive Test Mean?

Interpreting a Negative-Then-Positive Chlamydia Test

A negative test followed by a positive one 3 months later has two main explanations:

1. New Acquisition (Most Likely)

• Infection occurred after the first test.
• Window period: NAAT detects Chlamydia ~1-2 weeks post-exposure. A test at 3 months would catch any exposure in that interval.

2. False Negative on First Test

• Low bacterial load (early infection, partial treatment).
• Improper sample collection (wrong site, insufficient cells).
• Test inhibition (blood, mucus, antibiotics).
• Test sensitivity <100% (NAAT ~95-98%).

Less Likely

• Persistent untreated infection from before first test (would likely have been detected).
• Reinfection from untreated partner.

Next Steps

• Treat current infection (azithromycin 1g or doxycycline 7 days).
• Notify partners from last 60 days (or last partner if >60 days).
• Retest in 3 months (reinfection rate high).
• Test for other STIs (HIV, syphilis, gonorrhea).

Timing matters: a negative test only rules out infection at that moment.