The halo test is something I learned many years ago that is useful for determining whether or not there is cerebral spinal fluid in blood from a head wound. This, in turn gives the provider some idea of the severity and extent of a head wound.

The test is simple to do: Take a clean piece of white gauze (with a gloved hand) and dip it in the blood coming from a patients head wound. This may be from the ear, nose or open head wound. The test works because cerebral spinal fluid is less viscous than ordinary blood. The cerebral spinal fluid will wick into the gauze faster than the blood will. This causes a "halo" or yellowish ring around the blood spot on your gauze. 

The photo to the left is of me directing an accident scene - car vs man on a bicycle. This man was hit pretty hard by the car and had secondary trauma from impact with the road. He had blood coming from his ears and nose. The halo test was positive in this case on blood coming from his left ear. 

There are critics of this test that say it doesn't work, but I've seen it work on the street. It's certainly not the best test available to a medical provider in a hospital setting for this sort of thing and it can't be considered definitive, but it is an acceptable "field" test for open skull fractures in my opinion. It's completely non-invasive and takes only a few seconds to perform. In a profession where reading the signs and piecing together clues is important, this test is a welcome addition to my tool box.