Blood stains are the most challenging type of stains to remove from a carpet. The moment you spot even a drop of blood on your carpet, clean it right away. The fresher the stain, the easier it is to clean. Blood is made up of iron and proteins. Coagulation of blood makes it harder to remove a dried stain. Water is one of the best cleaning agents, but never use hot water on a blood stain because the stain sets faster with the help of hot water.
Rinse the stain with cold water. Let the carpet absorb the water a little. Use absorbent rags or a white cloth to blot the stain. Do not try to wipe the stain, just blot it. You can even use a shop-vac to suck the solution out of the carpet. Repeat this process until the stain is removed.
Mix a solution of 1 tbsp household ammonia in 1 cup of cold water. Blot the fresh blood stain with a rag or a sponge. Spray the ammonia-water solution onto the stain until it saturates. After a few minutes, blot up the liquid and stain with a clean rag. Be careful while using this solution. Ammonia is hazardous to health and should not be inhaled.
Use any good laundry cleaner or dish washing detergent on the stain. Rub the liquid into the stain with the help of your finger. Be very careful not to spread the blood stain. Allow the cleaner to set for a few minutes, and use a piece of sponge to scrub the blood. Soak the area with cold water and blot it using a clean rag or cotton cloth.
Repeat the process of soaking the stain with cold water and blotting it with a rag or a sponge until no more blood comes out onto the rag. Blot the excess water.
Leave the fan blowing over the area to allow it to dry. Keep a towel or a number of rags over the area, and put something heavy on top of them. Let the remaining liquid blot up, and keep replacing the towel or rags, if required.
If the cold water sponging method does not remove the stain completely, use club soda to remove the remaining stain. Club soda can be helpful with stains on thin carpets and fabrics.