A specimen for microscopic examination with the use of compound microscope is blood. Blood for cultures must be collected with special care because microbes, especially staphylococci, are present on the surface of the skin and within its superficial layers.

A blood culture is usually taken by venipuncture. Venipuncture is done first by painting the skin over the veins in the bend of the elbow with tincture of iodine or other suitable disinfectant like 70 percent alcohol. Some persons may be allergic to iodine. Make a second application of tincture of iodine or 70 percent alcohol to thoroughly sterilize the area. Then compress the area. Secure a tourniquet, but not too tightly, around the arm just above the elbow. Ask the patient to close and open the hand several times.

Puncture a prominent vein with the use of a 20 or 21 gauge needle to which may be attached a 20 ml. syringe or vacutainer. Vacutainer is a specially designed holder for a vacuum tube. Withdraw from 10 to 15 ml. of blood or whatever amount is indicated for the particular laboratory test.

Add the blood directly to culture medium or place it in a sterile bottle containing sodium citrate to prevent coagulation. Bring the sample to the laboratory where it can be distributed to suitable culture media. To prevent contamination, remove the needle from the syringe before expelling the blood into culture media.

Bacteremia is often transient. If the blood culture is not collected at the proper time, the etiologic organism of the disease may not be found. Since the advent of the antimicrobial drugs, positive blood cultures are fewer. Even one dose of an antimicrobial drug before the culture is taken may mask the infec¬tion. The bloodstream in health is free of microorganisms.