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It is good to use some sort of protection when you don't wont to be pregnant.
test is nearly 98% accurate. It should be done at least week after the missed period. If done earlier there are chances of false negative results.
I advice you repeat the test. As you have mentioned you had your last period 17th April, the date of test should be at least week from 17th April. It is better to do the test with early morning urine sample.
Hope i have provided the information you needed.