Interpretation of tube agglutination test for bovine brucellosis with turbidimetric readings and international unit

The tube agglutination test has been used for bovine brucellosis diagnosis in Korea since middle 1950s. The reported high specificity was its value in eradication program. However, the reading of reaction mostly depends on personal experience, thus here we report a way to improve accuracy and uniformity of reading. The tube agglutination was conducted according to the protocol provided by Korean Ministry of Agriculture and Forestry. The intensity of reaction was measured by spectrophotometer. The relationship between turbidity and percentage clearing was generally in direct proportion and linear. The correspondent percent transmittance at 75, 50, and 25% clearing were 91, 82, and 73, respectively. Then, the degree of percentage of clearing at given international unit was measured. With about 1.5 unit of serum, the maximum percentage clearing was observed. The international unit showing 25, 50, and 75 percentage clearing were 0.61, 0.83 and 1.35, respectively. Based on the information obtained using international standard serum, the calculation of international unit of test serum was available. According to the protocol for bovine brucellosis diagnosis which provided by Korean Ministry of Agriculture and Forestry, the available range of detectable international unit was between 15 and 538. And the corresponding international units for suspicious case ranged between 42 and 127. Of the 35 sera from B abortus infected cattle, about half of them had more than 538 international units. Collectively, the reading of turbidity using spectrophotometer and application to international unit improved accuracy and uniformity of reading.