Use of twice-distilled water or addition of normal equine serum, preheated at 56 degrees C for 30 min and adsorbed with an adsorbent from the diagnostic kit, to normal saline (1 ml of serum per 100 ml of the saline) helps eliminate spontaneous agglutination of the tested diagnosticum during the detection of HBsAg with the use of reverse passive hemagglutination test, when the experiments are carried out in areas with high concentrations of salts and organic substances in the water.