Most of the present generation of test kits are highly accurate and have now reached a very high level of sensitivity and specificity. If correctly used, false-positive or false-negative results due to interfering materials are extremely unlikely. However a lot also depends on the timing of the test. Most kits are sensitive enough to detect a pregnancy by the time of the next day of expected periods.
Around 10 days after a missed period if the HPT tests negative it is extremely unlikely that there could be a pregnancy.
Delayed menses can be caused by a variety of reasons other than pregnancy. Hormonal imbalances, weight fluctuations, thyroid illness, polycystic ovaries etc. all causes of delayed menses.I don't think in your case the reason for the delayed periods could be pregnancy especially as your pregnancy tests have all tested negative. To know the exact reason for the delayed menses you will have to consult a gynecologist or a local doctor. The doctor will examine you and will probably advice certain hormonal evaluations, ultrasound examinations to determine the cause of delayed periods.
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