Pregnancy test makers claim that their results are 99% reliable, however, this percentage is only a comparison with other manufacturers, not with regard to the number of pregnancies they detect or the certainty of your specific case according to the Administration of Medicines and Food (FDA). In fact, the factors that determine the reliability of the results are the moment you take it, the sensitivity of the test and its correct use.
If you take it before you have a delay in your menstrual period, the reliability of a negative result may be less than 50%. This information can be found in the attached document that has your pregnancy test. To have reliable results, wait until after the delay in the menstrual cycle and confirm it by taking another test one week later Women Health
Is it possible to have a false negative?
Sometimes you may get a false negative result. This can happen for several reasons;
You took the test very early and your hCG hormone levels (known as the pregnancy hormone) were not detected.
The test is not sensitive enough to detect your hCG level.
There was some mistake in collecting the urine sample.
You assume a negative result because of the difficulty to read it.
Your urine was very thin. If you drink too much fluid before taking your test, the amount of hCG is diluted and it becomes more difficult to detect.
Is it possible to have a false positive pregnancy test?
Obtaining a false positive pregnancy test result is less common, but false positive pregnancy test may occur in the following cases;
In women under fertility treatment.
Women who have been pregnant recently, even if an early loss occurred or the pregnancy did not come to term.
If blood or protein is found in your urine.
Some tranquilizers may alter the results.
You read the results after the time indicated by the manufacturer.
If you have difficulty reading the results or it is difficult to identify the color or intensity of the lines in the result window.
Some diseases can raise your hCG levels such as certain cancers or ovarian cysts.
Menopause can cause hormonal imbalance that can alter the results of your test.
Pregnancy tests that are defective or that exceed their expiration date.
A chemical pregnancy, which is when the ovum is fertilized but does not thrive. This is, in essence, a pregnancy that never occurred, as it ends in a menstrual period, although some doctors consider it an early loss.