Relationships Among the Fatty Acid Profiles of Plasma, Red Blood Cells, and Dried Blood Spots

The fatty acid (FA) composition of red blood cells (RBC) is often used as a proxy for the FA profile of other body tissues and as a health parameter in many species. However, the collection, isolation, and storage of RBC may be difficult in some circumstances. Dried blood spot (DBS) cards have been validated in other species to conveniently measure the FA profile of animals. The objectives of this study were to examine relationships among plasma, RBC, and DBS FA profiles and to validate the use of DBS to predict the composition of RBC. Data from 4 studies were combined to generate a dataset containing 134 blood samples from which we collected plasma, RBC, and DBS. Each blood compartment was analyzed for 24 FA. The samples in the dataset were placed into 5 groups based on horse diet: pasture only (n = 24); hay and grain only (n = 12); pasture and different levels of long-chain polyunsaturated fatty acid (LC-PUFA) supplementation (n = 60); pasture, soy oil, and different levels of LC-PUFA (n = 22); and hay and grain with different levels of LC-PUFA (n = 16). Simple linear regression was used to generate equations to predict RBC FA from DBS FA. ANOVA was used to evaluate the effects of diet and collection method (plasma, RBC, DBS) on FA composition. The percent of RBC omega-6 FA was greatest in horses fed hay and grain (mean 39.8%) and lowest in all groups consuming pasture (mean 36.7%; SEM 0.3%; P < 0.0001), and the percent of RBC omega-3 FA was greatest in the pasture plus LC-PUFA group (mean 4.8%) and lowest in the hay and grain group (mean 3.0%; SEM 0.1%; P < 0.0001). Because DBS comprises RBC and plasma, the FA composition of DBS is often intermediate between RBC and plasma values, such as for total omega-6, total omega-3, and monounsaturated FA (P < 0.05; Table 1), but RBC and DBS values were similar for EPA and saturated FA (P > 0.05). Despite some differences in overall mean values between RBC and DBS, significant relationships were found using regression. The relationship with the greatest R² value between RBC and DBS was for DHA + EPA (R² = 0.83; P < 0.0001), termed the omega-3 index. This index is used in humans as an indicator of heart disease risk, but this index could be useful in horses to measure other health outcomes with more research.

Pagan, J.D., R.W. Springer, and A.L. Fowler. 2025. Relationships among the fatty acid profiles of plasma, red blood cells, and dried blood spots. Journal of Equine Veterinary Science 148:105559.