New polymorphisms in the nucleic acid sequences of the DNA polymerase/reverse transcriptase open reading frame and viral surface antigen open reading frame of the hepatitis B virus are reported. In particular, the present invention relates to the mutation YMDD→YSDD in the HBV reverse transcriptase domain and to the W196V mutation in the small HBV viral surface antigen. Said polymorphisms are affecting the detection of drug resistance mutations by genotypic methods and diagnostic kits based thereon. The present invention relates to methods and diagnostic kits for detection of a HBV virus comprising said nucleic acid polymorphisms. In particular, those methods utilizing oligonucleotides capable of hybridizing to said HBV nucleic acid polymorphisms are envisaged.