Can hcg analyses show wrong answer in blood?

Pregnancy test: Like any test there can be false positives and false negatives. An example, if a pregnant woman had her blood drawn at the same time as a nonpregnant woman, but there samples got switched, this could cause a false negative and false positive, respectively. There are also some substances, antibodies, which can occassionaly cause false positives and negatives. These are unusual situations.