Saline agglutination tests (SATs) are commonly used in the diagnosis of canine immune-mediated hemolytic anemia (IMHA), but frequent false positive results have been reported using a 1:1 saline to blood ratio. To determine if increased saline to blood ratios improve specificity, SATs were performed at a saline to blood ratio of 1:1, 4:1, 9:1 and 49:1 for 104 dogs with hematocrits ≤ 30%. SATs were considered positive if at least 1 of 5 randomly selected x40 objective fields contained ≥ 1 aggregate of ≥ 4 erythrocytes or ≥ 2 aggregates of 3 erythrocytes. Of the 104 dogs, 5 were classified as affected by IMHA based on a positive direct antiglobulin gel test and clinical evidence of hemolysis. These criteria were not met in 96/104 dogs classified as unaffected by IMHA and 3/104 were excluded because IMHA status was unclear. Specificity for IMHA was 32% (95% confidence interval 22-43) for 1:1, 56% (45-66) for 4:1, 70% (60-79) for 9:1 and 95% (88-98%) for 49:1. Sensitivity was 100% (40-100) for 1:1 and 4:1 and 80% (30-100) for 9:1 and 49:1. Positive predictive value increased from 7% (6-8) for 1:1 to 44% (23-68) for 49:1. Negative predictive value was 100% at 1:1 and 4:1 and 99% at 9:1 and 49:1 (92-99). Diagnostic accuracy was 35% (25-46) for 1:1, 58% (47-67) for 4:1, 70% (60-79) for 9:1 and 94% (88-98) for 49:1. Based on these results, SATs should be performed at a dilution of at least 49:1 to improve specificity for diagnosis of IMHA.
Learning Objectives:
Upon completion, participants will be able to select an appropriate dilution for preparing saline agglutination tests.
Upon completion, participants will be able to interpret the likely clinical significance of a positive saline agglutination test.
Upon completion, participants will be able to interpret the likely clinical significance of a negative saline agglutination test.
