A device for separating blood plasma from whole blood includes a first reservoir and a second reservoir. The first reservoir is configured to receive a sample of whole blood including red blood cells and includes a collection region and a constricted region. The second reservoir is fluidically connected to the constricted region of the first reservoir, such that, responsive to centrifugal force applied to the device, the sample of whole blood disposed within the first reservoir separates into a first fraction and a second fraction. The first fraction is located in the collection region and includes blood plasma from which substantially all red blood cells have been removed. The second fraction is located in the second reservoir and includes blood plasma and red blood cells that have been removed from the first fraction by the centrifugal force. The constricted region inhibits the second fraction from entering the collection region.