I kNOW that
blood test is more accurate than urine
test and can detect pregnancy after 24 from implantation while urine test can give false positive results and false positive may be due to:
1-chemical pregnancy
2-missed reaction time(too long to read)
3-chemical interference from FERTILITY drugs having HCG ,medication like trankilizer phenothiazine,contamination with blood or gross amount of protien,
4-evaporation line,you see faint evaporation line as the urine crosses the test in the test are