How long can a test strip be used to detect pregnancy?

Patient's question:

Generally, it takes about 7 days after the period to detect pregnancy using a test kit. Adding the fact that ovulation occurs about 14 days before the period, it would seem that pregnancy should be detectable around 21 days after conception. However, some sources say that pregnancy can be detected as early as 7 to 11 days after conception using a test kit. Why is that?
My last period was on March 28th, and I tested positive for a weak line on May 15th. Can you help me calculate which days are most likely for conception? Additional question: In the two months before pregnancy in March, my menstrual cycle was about 34 days long. Last period was March 28th, tested positive for a weak line on May 15th, and on May 25th...

Doctor's answer:

Hello, for everyone, there is a slight difference in the timing of testing after having sex a few days with early pregnancy test strips, because shortly after conception, due to the low concentration of hCG in the blood, it is often difficult to detect or shows a weak positive result. As time passes, the hCG concentration increases, allowing the urine test result to show positive. The earliest detection can be possible 10 days after conception.
At the same time, gynecological experts point out that when women perform pregnancy self-tests at home without any external guidance, the accuracy of the test results generally only reaches 50% to 75%. If this test is conducted in a laboratory under the supervision of a doctor, the doctor can ensure that the test strips are working properly, and the woman can use the test strips correctly according to the instructions, making the test accuracy potentially close to 100%.

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