Pregnancy kits test yung hcg level natin which is high during pregnancy. However, pag masyadong early tayo ng pt, possible na hindi pa ma detect kaya negative. Minsan depende din sa brand ng pt na ginamit. After several days, pag inulit, dun lang nagpo-positive, IF you are really pregnant. But pregnancy testing thru blood test almost never fail, regardless kung early stage man yan or hindi.