I got a quant hcg blood test 16 weeks after sexual encounter and the result was <1.0miu/ml hcg, my doctor said that was negative, is it safe to say im 100% not pregnant?

Tests are accurate: Pregnancy tests are very accurate and not affected by foods, herbs, common illnesses, sexually transmitted diseases, vaginal spotting, or medications. A test gives a reliable result by 3 weeks after sex (2-3 weeks after when conception might have happened) or 1 week after missing a period. If done sooner, a negative result may be due to testing too early such as less than 2 weeks after conception.