Cold or cool soak with Biz or a detergent that contains enzymes (which most do except the cheapies, check the ingredient list).

There'll be some more experts coming on here sooner or later, cottonpenny. In the meantime, ditto what dadoes said.

You don't say if the sheets are white or colored. If white, you could use straight hydrogen peroxide. People who work in medical laboratories use it for blood stains on their lab coats. You can google "using hydrogen peroxide to remove blood stains" and find lots of info. Peroxide is a bleach, so wouldn't be good on colored sheets. Cold water soak and cold washing should do the trick. If the stain if very fresh, you might not even need the enzyme which might be hard on some fabrics.

Definitely rinse and soak in cold water first, and use it with any of the above-suggested soaking products.

Warm water only, no chemical. Soluble at 89 degrees. Cold or hot will set the stain.

I use a product called "Awesome" that I purchase at the Dollar Tree store in the states. It is unbelievable what it can get out. It has gotten dried blood out of my daughter's mattress pad with absolutely no trouble at all. Also, another product I use and swear by is the Tide Booster in the cylindrical orange bottle. I've been using it for about two years now and they are just starting to sell it in Canada.

I also pre-soak our white socks in Awesome and hot water every few weeks and they whiten right up.

Warm or hot water will set the stain. Cold water is what you need to use to get blood stains out of laundry.

I've always put such laundry in the washer ASAP, fill it up with cold water, let it agitate for a few minutes, then pause the washer and let the laundry soak.

Since you're talking about bed sheets, I would get them soaking first thing in the morning and let them soak as long as possible -- like late afternoon if you need them that evening.

Then let that water out of the washer and fill with fresh cold water with a good enzyme detergent to wash. Hydrogen peroxide is cheap and helps too, but I don't usually need it if I soak as described above.

HTH

Fill a small spray bottle with hydrogen peroxide and spray the blood. It will "eat" the stain if you saturate it enough and is safe for the fabric. It's what we use in the spay/neuter clinic on our scrubs. If there is a lot of blood, and it's become more set overnight, you might want to investigate going to a beauty supply house for hair processing peroxide which is available in stronger concentrations than the 3% sold in drug stores. It's in a lotion/gel form and won'be be able to go through a spray bottle, but a ketchup or mustard squeeze bottle will work just fine for that.

After the blood is "eaten", put the sheets in cold water with an enzyme detergent. Then inspect them before they go into the dryer. If any stains remain, treat them again with the peroxide. If any bit of the stain is still there after washing and it's put into the dryer, you'll never get it out.

Rub out the affected area of cloth by applying solution of ammonia and water before laundering.

I use 10 year old Asko 220V machine. I treat blood with Resolve spray (I think is the name) which used to be Spray and Wash. Then I wash with Tide HE plus Oxyclean at 160F plus temp. Never had any problems with stains. Oxyclean is predomantly Hydrogen peroxide as noted above. On tough blood stains, I have put oxyclean with mixed with water directly on the stain and soaked over night. I often soak my kitchen towels in Oxyclean and it is amazing the what comes out of the soaking...

I am not sure if I helped you or not since most people cannot wash their clothes at a higher temp. This is why clothes were "boiled" in the olden days, to get the stains out....

The Asko washer always starts with cold water. Then the water is heated to a temp set by the machine (up to 200F). The machine cycle takes about 2 hours. No kidding. The warm/hot water sets in the stains before the chemicals have had the chance to do its work. Any type of protein stains get set into the fabric from the hot water.

The kitchen and household cleaner called 409. Its amazing! Spray a little on colored or white sheets. Rub the fabric with itself until the stain starts to lift a little then wash it with regular detergent in cold water and the stain will come right out