The contamination of blood cultures is a common problem with important clinical consequences because blood cultures are critical for the diagnosis and management of bloodstream infections. Many blood cultures have produced false-positive results due to the inadequate preparation of the skin for blood collection and contamination of the blood culture may confuse the interpretation of the results. Furthermore, such contamination is related to poor patient care and it may lead to a prolonged hospitalization stay, unnecessary antibiotic use, increased tests, and other healthcare costs [1,2]. The most common contaminants are coagulase-negative staphylococci (CNS). However, these organisms can become significant pathogens in certain cases. Studies found that 25 to 37% of cultures containing CNS showed significant bacteremia [1,3,4]. Another study showed that approximately 50% of the patients with false-positive cultures still received antibiotics although the physicians were quite certain that CNS was contaminant [1]. These findings suggest that there are diagnostic and therapeutic difficulties when these organisms are isolated. The selection of skin antiseptic is a critical step in the prevention of blood culture contamination. We reviewed the effectiveness of different skin antiseptic agents with regard to the rate of false-positive blood culture results. Several studies have revealed that chlorhexidine is superior to povidone-iodine or alcohol alone for skin preparation for blood sample collection [5]. It has been suggested that education of health-care professionals about adequate skin preparation for blood sample collection may be the key to reduce blood culture contamination [6].

The volume of blood drawn for culture is the most important variable in detecting bacteremia or fungemia [7,8]. An inadequate volume (e.g., too little or too much volume) of blood culture prevents pathogens from attaining correct isolation. For adult patients, the recommended volume for blood culture is from 20 to 30 ml for each venipuncture, with the implication that culturing a higher volume of blood containing small numbers of bacteria will improve the recovery of the bacteria [9]. Indeed, the effectiveness of blood cultures increases in accordance with the volume of blood collected [4,8,10]. Because the bacterial number is less than 1 CFU (colony forming unit)/mL in above than half of adulthood bacteremia [11], it is imperative to design a practical blood culture protocol with a large amount of blood volume. However, a multicenter study revealed that the average blood volume for each set was 7.7 mL at 9 university-affiliated hospitals in Korea [12].

The correct use of disinfectants is essential because improper skin preparation is the main cause of contamination. In addition, the sampling of an adequate volume of blood is also mandatory for a successful blood culture. In this study, we compared the efficacy of two skin antiseptics, povidone-iodine and chlorhexidine-alcohol, in preventing blood culture contamination and we further analyzed the efficiency of the increased blood volume recommended by the CLSI guidelines [9].

Blood culture is one of the most common laboratory tests implemented for the diagnosis of bloodstream infections. However, the problem of skin contamination has been known to be widespread and the consequences due to contaminated cultures are not negligible. Up to 50% of the positive cultures may be considered contaminated [13] and previous studies have reported that 0.6 to 6.25% of blood cultures were contaminated [1,5]. The proper preparation of the skin prior to obtaining the blood for culture is an imperative step because contaminated blood cultures are problematic to interpret [14]. Some studies found that the costs caused by contaminated cultures were significantly higher due to antibiotic use, total laboratory costs, and microbiology costs than the cost with negative cultures [1,2]. Beyond the financial costs, the suppression of normal flora and the development of antimicrobial resistance due to unnecessary antibiotic usage would be harmful for patients. According to studies reviewing the appropriate skin antiseptic agents to reduce blood culture contamination, no clear evidence suggests the best skin antiseptic agent to prevent false-positive blood culture results [15]. In previous studies, a number of antiseptics have been used for skin preparation for blood cultures, including alcohol, povidone-iodine, tincture of iodine, and chlorhexidine. Povidone-iodine combined with alcohol preparation has been most commonly used in Korea [12], although the other studies have found tincture of iodine to be more effective [2,16]. There are also evidences to suggest less contamination with an alcoholic solution of chlorhexidine compared to povidone-iodine [5,17].

In GNUH, 10% povidone-iodine was routinely used for the preparation of the skin for blood culture until a new protocol was implemented. In the second period, chlorhexidine-alcohol was used instead of povidone-iodine for skin preparation and we found higher rates of contamination when using 0.5% chlorhexidine-alcohol versus 10% povidone-iodine. These results are contrary to the previous findings [5]. More diverse skin contaminants were recovered using chlorhexidine-alcohol, suggesting this disinfectant might be less effective at removing CNS and other skin flora compared to povidone-iodine. However, these results should be interpreted carefully because the study period was not of sufficient duration and the procedure of skin disinfection had not been monitored. Usage of 20 mL syringe which might be more inconvenient than 10 mL syringe, educational intervention before the second period, and short antiseptic drying time of 20-30 seconds might have affected to our results. Alcohol-based disinfectants should be applied by vigorous friction [18]. It is possible that the increased blood volume in the second period might have contributed to the favorable growth of skin contaminants. If the skin contamination rates were already very low, as at our institution, it might have been very difficult to obtain lower incidences. As the recommended contamination rate is ≤3% according to the CLSI guidelines [9], the skin contamination rates with povidone-iodine (1.1%) and chlorhexidine-alcohol (2.0%) seem satisfactory. Because skin contamination in blood culture is critical, this issue should be further studied more in detail. For a more concrete conclusion of the better skin antiseptic, longer study duration, inpatients group, and diverse phlebotomy teams (phlebotomists, physicians, and nurses) with a randomized trial should be designed.

The blood volume is the key parameter for successful blood culture. The positive rates between the 10 mL and 20 mL collection volume per set were significantly different (12.5% versus 17.1%, respectively). However, this result should be carefully interpreted, as the patient population is different between the two periods. Comparing different volumes (such as 10 mL versus 5 mL per bottle) from the same patient should be implemented to exclude these variables. Considering the positive rates recommended by the CLSI guidelines (of 6-12%), the blood culture performance is evaluated as excellent in the Emergency Department at our institution and the phlebotomists followed the new protocol well in terms of the amount of blood collection. However, continuous monitoring and education would be needed for the correct amount of blood volume, because our data showed a little behind the required blood volume (~15 mL for 20 mL/set protocol). We suggest that medical institutions should follow the CLSI guidelines, especially in terms of the optimal sampling amount (20 mL/set), because most of the institutions in Korea currently adopt a 10 mL/set protocol [19]. More than 20 mL/set would not be justifiable, as the broth occupies 40-45 mL of the broth volume and the optimal ratio of blood to broth would be 1 : 5-10 [9]. Interestingly, polymicrobial infections were more common with the 20 mL/set blood collection protocol (6.0% versus 1.9%); however, the conclusion that a higher blood volume collected caused the polymicrobial infections cannot be drawn with our limited data.

Our findings should be interpreted with regard to several limitations. First, the populations studied were different across the time period; the use of different time frames is a potential problem. Second, seasonal factors were not considered; therefore, it is possible that the contamination rates observed were due to changes in the patient populations or seasonal trends. Third, the Emergency Department is a specialized area where emergency situations are common. Although the phlebotomists received education on aseptic techniques twice, this may have been insufficient to change their behaviors satisfactorily. Fourth, this study was performed at a single center during a short period of time; therefore, the results may not be applicable to the other institutions universally. Fifth, there was no wash-out period included and we believe that 1-2 months of a wash-out period after changing the skin disinfectant would be optimal. An analysis of the data for the first 2 months in the second period did not show a remarkable difference from the rest of the period, thus we included the data. Sixth, the time to positive detection was not measured during the study period. Generally, normal skin flora becomes positive after more than 3 to 5 days of incubation; whereas, most of the true pathogens grow within a day [3]. Thus, the measurement of time to positive detection might provide more clues to differentiate the true pathogens from the skin contaminants. The medical records were not reviewed to differentiate the skin contaminants from the true pathogens. Besides, the contamination rate of the blood culture could be different according to the definition of the skin contaminants. Lastly, this is not a randomized, controlled study, which could provide a higher reliability of the results [2,5,15].

In conclusion, both disinfectants seemed acceptable for the preparation of skin for blood culture collection, although chlorhexidine-alcohol had a higher contamination rate than povidone-iodine. The positive rate of blood culture was in accordance with the amount of sample collected. Continuous education and monitoring is needed for the good performance of blood culture.