Quantitative Microbiologic Study of Blood Salvaged by Intraoperative Membrane Filtration

Continuous-flow intraoperative red blood cell salvage is a safe and effective method to reduce the need for autologous blood donation and homologous blood transfusion in both elective and emergency surgery.1–6 However, because an expensive machine and a dedicated perfusionist are required, continuous-flow blood salvage is inaccessable to many mid-sized and small hospitals.

An alternative system using membrane filtration has been developed that is less expensive because it obviates the need for a continuous-flow machine and a staff of skilled perfusionists. Because the membrane filtration system has not been studied for bacterial contamination, this study prospectively evaluated the microbiologic quality of the harvested blood.

Blood salvage was achieved using Boehringer Laboratories' (Norristown, Pa) Autovac Blood Processing System, adhering to the manufacturer's directions.7 Briefly, the system consists of a single patient–use membrane filtration device into which shed blood is aspirated using standard operating room suction (Figure). Clotting is prevented by citrate anticoagulation. The blood is washed in the filter device using sterile saline before the red blood cells are rapidly separated using vacuum. A new filter is used after every second unit collected. The resultant washed erythrocytes are available for immediate reinfusion or may be held up to 6 hours at room temperature or up to 24 hours provided storage at 1°C to 6°C is begun within 6 hours of initiating the collection.8 

To determine bacterial colony-forming units/mL (CFU/mL) of blood, 1 mL of salvaged blood was removed by sterile technique from the filter system prior to reinfusion of the blood back into a patient. The blood was added to melted tryptic soy agar (cooled to 42°C–45°C), mixed, and poured into a sterile petri dish.9 

After cooling, the inverted plate was incubated at 35°C. Plates were examined for growth at 24 and 48 hours, and then daily before being discarded after 5 days. Colonies were examined by Gram stain, and organisms were identified by catalase and coagulase testing. The number of colonies per plate equaled CFU/mL.9 

All patients underwent elective spinal surgical procedures, predominantly lumbar fusions with grafts and/or instrumentation. Immediately preoperatively, patients received 1 g oxacillin and 80 to 100 mg intravenous gentamycin; an additional gram of intravenous oxacillin was administered every 4 hours during the surgery. The experimental protocol was approved by the hospital's Institutional Review Committee. Medical records were reviewed to identify transfusion-related complications.

Eighteen patients (13 men, 5 women), aged 19 to 87 years (mean age 49 years), participated in the study. Thirty-one units of blood were salvaged (average 1.7 units/patient, average volume/unit 347 mL). Ten patients had only 1 unit collected because their blood loss was less than anticipated preoperatively. Two units were recovered from 5 patients, 3 units from 1 patient, and 4 units from 2 patients. Of the 31 units recovered, 12 exhibited no growth, 17 grew 1 CFU/mL, 1 grew 2 CFU/mL, and 1 grew 5 CFU/mL (Table). Of the 19 units manifesting growth, 16 yielded only diphtheroids, 1 unit grew coagulase-negative staphylococci, 1 unit grew diphtheroids and micrococci, and 1 unit grew diphtheroids and coagulase-negative staphylococci.

No intraoperative or postoperative complications were attributable to the salvage process or reinfusion of salvaged blood, and no patient experienced postoperative sepsis.

Intraoperative blood salvage is a valuable technique for reducing the need for autologous blood donation for elective surgery and for homologous blood in emergency surgery. Salvaged blood eliminates the risks of infectious diseases, such as hepatitis and human immunodeficiency virus, and eliminates transfusion reactions due to donor antigens.

For salvaged blood to be acceptable, it must not be contaminated with microorganisms during collection and processing. At least 10 prior studies incorporating more than 800 patients have evaluated the incidence of bacterial contamination in blood recovered by continuous-flow intraoperative blood salvage.10–18 The frequency of positive blood cultures in these reports ranged from 3% to 97%. When quantitative cultures were performed, growth varied from less than 1 CFU/mL to fewer than 10 CFU/mL, with most producing fewer than 5 CFU/mL.12–15 Organisms recovered were predominantly coagulase-negative staphylococci and diphtheroids, but rarely a Streptococcus viridans, Neisseria species, Haemophilus species, Moraxella species, or gram-negative bacillus was identified. No patient was reported to have acquired sepsis from low-level (≤5 CFU/mL) contaminated salvaged blood.10–18 

Although this study population comprised only 18 patients, the results of this investigation support the thesis that blood salvaged by the Autovac Blood Processing System is microbiologically equivalent to that obtained by continuous-flow intraoperative blood salvage.10–18 Twelve of these units had no microbiological growth. Seventeen units yielded 1 CFU/mL, 1 unit 2 CFU/mL, and 1 unit 5 CFU/mL, all of which are within the maximum 5 CFU/mL that the literature suggests is safe for reinfusion.10–18 The only organisms recovered were diphtheroids, coagulase-negative staphylococci, and micrococci. None of our patients experienced posttransfusion complications attributable to the membrane-filtered blood.

Since completion of this study in June 1999, the intraoperative membrane filtration system has been used in our hospital for 17 additional spinal surgical and vascular surgical patients without any evidence of clinically significant blood contamination or adverse patient response.

The findings of this study show that intraoperative membrane-filtered salvaged blood is microbiologically equivalent to blood salvaged by continuous flow. An additional benefit of membrane filtration is its relatively low operational cost, which makes it accessible to small and medium-sized hospitals.