Contamination of blood cultures is not infrequent, causing 35 to 50 percent of all positive blood cultures. Adequate disinfection of the skin is a crucial step in avoiding false-positive results. Little and colleagues conducted a randomized trial to compare 10 percent povidone-iodine with 2 percent iodine tincture for skin disinfection before obtaining blood cultures.

Antecubital venipuncture samples were obtained with the patients randomized to have the skin site disinfected with one of the two solutions. Before the iodine tincture was applied, a 70 percent isopropyl alcohol applicator was applied for one minute. The study solution was then allowed to dry for two minutes before phlebotomy proceeded. Each culture set consisted of 10 mL of blood inoculated into two bottles. Blood cultures were considered positive if either bottle grew bacteria or fungus. Positive cultures were then classified as true positive or false positive. False-positive cultures were those that grew skin microflora.

The study included 3,851 cultures: 1,947 were randomized to the povidoneiodine group and 1,904 to the iodine tincture group. Nearly 10 percent of all blood cultures were positive; one-third (32 percent) of these were determined to be false positives. The povidone-iodine group was significantly more likely to have skin contamination (3.8 percent of that group) than the iodine tincture group (2.4 percent). This finding represents a 36 percent reduction in blood culture contamination rate. The rate of true-positive cultures was not affected.

The authors speculate that a rapid drying time of the antiseptic solution may be critical in determining why the iodine tincture was more successful at reducing the rate of skin contamination. The two- to three-minute drying time recommended after use of povidone-iodine solution probably does not often happen in practice. Because the iodine tincture dries more rapidly than povidone-iodine, it may act more rapidly to decrease skin microbe activity.