PREGNANXY TEST / URINE TEST STRIP

CJ
Okay so i just had a pregnacy scare. Boyfriend and I use protection and on the odd occasion we go without (usually unplanned). but more often that not we do! My period was due 4 days ago and still no sign! My friend let me use these urine test strips and it came up negative (thank god) but i was wondering if anyone knows how reliable they actually are? TIA!