Took hpt on the 16th day and 18th day of the last sexual contact using withdrawal and results were negative. is it accurate or i tested too soon?

Test is accurate: The test itself is relatively accurate. However, the timing of the test is even more important. The appropriate time is within few days from your missed period or about 2 weeks after last ovulation. Time from last sexual encounter is meaningless unless you understand that pregnancy takes place only after ovulation and ovulation takes place about 2 weeks from the start of your last period .