33
DIAGN MICROBIOL INFECT DIS 1991;14:33-35
LETTER TO THE EDITOR
Mycobacterial Cross-Contamination with the Modified BACTEC460 TB System Patrick R. Murray
The BACTEC460 TB System (BACTEC) is an excellent system for the detection of mycobacteria in clinical specimens. A number of well-controlled studies have documented the improved sensitivity and significant reduction of detection time with this system. Unfortunately, the design of the BACTEChas introduced problems with cross-contamination w h e n sampling bottles inoculated with clinical specimens. My laboratory (Vannier et al., 1988) and that of Witebsky and colleagues (Witebsky et al., 1988; Conville and Witebsky, 1989) have previously described this problem, and demonstrated that it was in part attributed to a defective needle heating cycle. Becton Dickinson Diagnostic Instrument Systems (BDDIS; Towson, MD) have attempted to correct this problem by extending the heating cycle to 80 sec. Although Conville and Witebsky (1989) did not observe further cross-contamination with the extended heating cycle, we have recently experienced such a problem. On May 24, a series of mycobacterial specimens were processed according to the established laboratory procedures, which includes inoculation of one BACTEC bottle for each culture, as well as Lowenstein-Jensen medium and Middlebrook 7Hll agar. Four days later, one of the specimens registered a From the Department of Clinical Microbiology, Barnes Hospital and Washington University School of Medidne, Saint Louis, Missouri. Address reprint requests to: Division of Laboratory Medicine, Washington University, School of Medicine, Saint Louis, MO 63110. Received July 27, 1990; revised and accepted August 14, 1990. © 1991 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010 0732-8893/91/$3.50
positive BACTEC reading (544 units) and subsequently was confirmed to be from a patient with tuberculosis. A second bottle registered a BACTEC reading of 63 units 14 days after the first positive bottle was reported. This bottle was separated from the first specimen by 11 culture bottles. All other media inoculated with this specimen remained negative. Three weeks after this bottle was discovered, another specimen registered a positive BACTECreading (999 units). This last bottle was positioned between the other two positive bottles (eight bottles after the original positive culture). As with the second positive culture, no mycobacterial growth was observed on the other inoculated media. All three isolates were subsequently confirmed to be Mycobacterium tuberculosis with the same antimicrobial susceptibility pattern. Neither patient with the potentially contaminated cultures had any clinical evidence of tuberculosis. Representatives of BDDIS were contacted when this problem was discovered. When one of their engineers examined the instrument, the needle heating block was found to function improperly. The needle should be heated to a temperature between 250 ° and 300°C; however, at no time did the temperature exceed 240°C. Despite this problem, the warning lights of the instrument did not indicate a malfunction. When this was discussed with the manufacturer, they stated that BDDIS do not claim that the BACTECsystem sterilizes the needle (S. Siddiqi, personal communication). Although this may be the BDDIS current position, their "Operation and Maintenance Manual" dearly refers to the needle heating system as a "needle sterilizer" and their field representatives commonly refer to the process as sterilization of the needle between culture bottle sampling.
34
In summary, a number of potentially serious problems exist with the BACTEC460 TB System. First, defective operation of the needle heating block can occur and can remain undetected by the system. At the present time, the only way that this function can be monitored is by the company. This should be corrected by a modification of the warning light component that will indicate when the heating block fails to reach the minimum specified temperature. Second, users of the BACTEC460 TB system should realize that cross-contamination between bottles can occur and be
Letter
separated by several uncontaminated bottles. Although this problem has been well documented, it may not be appreciated by most users. Finally, if BDDIS denies that their instrument sterilizes the sampling needle, this should be clearly stated in their product manual and further clarified by their field representatives. Moreover, if the manufacturer takes this position, then the liability of false-positive mycobacterial cultures will be shifted to the laboratory user. The legal ramifications of this should be carefully considered before a laboratory uses such a system.
REPLY
We appreciate Dr. Murray's concern about crosscontamination with the BACTECTB System. We realize that the occurrence of cross-contamination, though rare, is a serious problem, and when it occurs, it warrants immediate attention. The BACTEC 460 instrument has been in use for mycobacteriology for more than 10 years. The incidence of cross-contamination was brought to our attention only in 1987. Realizing the gravity of this problem, we began investigating solutions to these rare occurrences at that time. In the meantime, we felt an obligation to share this information with our BACTECusers. A bulletin was sent out (BACTECDATA, Nov. 1987) defining the problem--"cross-contamination occurs in the BACTEC 460 System when organisms from a truly positive vial are transferred via the needles into a subsequent negative vial or vials." In the same notification, we recommend specific preventive maintenance procedures and pointed out that "on occasion, faulty circuitry in the 460, that alters the needle heating efficiency, can contribute to the problem." To minimize further the potential for cross-contamination, a modification of the electronic boards that regulate the heating cycle was developed. This resulted in an increase in the needle heating time. Several instruments were fitted with these modified boards and subjected to rigorous testing in-house in order to assess whether cross-contamination could still occur. Having seen no evidence of cross-contamination, we were confident that this approach would take care of the problem. The new board was rigorously field-tested and was found to prevent crosscontamination effectively (Conville and Witebsky, 1987). A program of replacing old boards with the new ones began in 1989. Recently, however, some of the newly installed boards have been found to be faulty. They have not produced the necessary temperature for a sufficient duration of time. We are taking c o r -
rective measures. Presently, every new board is being checked thoroughly for temperature output before it is installed. We also plan to check all previously installed new boards. At selected sites we intend to implement a special temperature-monitoring device for a long-term study of the efficiency of the new board and needle heater. In addition, each of our field service engineers will soon have special equipment to monitor needle heater performance at the time of preventive maintenance visits. Concerning the issue of "needle heater" versus "needle sterilizer" which Dr. Murray has raised, initially, we incorrectly described the 460 needle heater as a needle sterilizer. In the early 1980s we began the modification of our operating manuals to describe the system more properly as a needle heater. At the same time, our sales representatives and field service engineers were made aware of this distinction. As early as Fall 1983, our BACTECNEWS, sent out to all BACTECusers, indicated that "needles should be considered contaminated and handled with appropriate caution at all times." "Prior to the removal of the needle set for autoclaving and cleaning, you may want to process an uninoculated vial as the final vial. This provides a heating cycle after the last patient specimen tested, which further reduces potential for accidental exposure." We do realize that malfunctions can occur in the BACTEC 460, as in any other instrument, and users should routinely monitor system performance. Falsepositivity, either due to cross-contamination or from environmental contamination, can occur in bacteriology. We have taken corrective measures with the BACTEC TB system and are continually looking into ways either to eliminate or minimize this occurrence further.
Salman H. Siddiqi, PhD Research Fellow B E C T O N D I C K I N S O N D I A G N O S T I C I N S T R U M E N T SYSTEMS
DIAGN MICROBIOL INFECT DIS 1991;14:33-35
LETTER TO THE EDITOR
Mycobacterial Cross-Contamination with the Modified BACTEC460 TB System Patrick R. Murray
The BACTEC460 TB System (BACTEC) is an excellent system for the detection of mycobacteria in clinical specimens. A number of well-controlled studies have documented the improved sensitivity and significant reduction of detection time with this system. Unfortunately, the design of the BACTEChas introduced problems with cross-contamination w h e n sampling bottles inoculated with clinical specimens. My laboratory (Vannier et al., 1988) and that of Witebsky and colleagues (Witebsky et al., 1988; Conville and Witebsky, 1989) have previously described this problem, and demonstrated that it was in part attributed to a defective needle heating cycle. Becton Dickinson Diagnostic Instrument Systems (BDDIS; Towson, MD) have attempted to correct this problem by extending the heating cycle to 80 sec. Although Conville and Witebsky (1989) did not observe further cross-contamination with the extended heating cycle, we have recently experienced such a problem. On May 24, a series of mycobacterial specimens were processed according to the established laboratory procedures, which includes inoculation of one BACTEC bottle for each culture, as well as Lowenstein-Jensen medium and Middlebrook 7Hll agar. Four days later, one of the specimens registered a From the Department of Clinical Microbiology, Barnes Hospital and Washington University School of Medidne, Saint Louis, Missouri. Address reprint requests to: Division of Laboratory Medicine, Washington University, School of Medicine, Saint Louis, MO 63110. Received July 27, 1990; revised and accepted August 14, 1990. © 1991 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010 0732-8893/91/$3.50
positive BACTEC reading (544 units) and subsequently was confirmed to be from a patient with tuberculosis. A second bottle registered a BACTEC reading of 63 units 14 days after the first positive bottle was reported. This bottle was separated from the first specimen by 11 culture bottles. All other media inoculated with this specimen remained negative. Three weeks after this bottle was discovered, another specimen registered a positive BACTECreading (999 units). This last bottle was positioned between the other two positive bottles (eight bottles after the original positive culture). As with the second positive culture, no mycobacterial growth was observed on the other inoculated media. All three isolates were subsequently confirmed to be Mycobacterium tuberculosis with the same antimicrobial susceptibility pattern. Neither patient with the potentially contaminated cultures had any clinical evidence of tuberculosis. Representatives of BDDIS were contacted when this problem was discovered. When one of their engineers examined the instrument, the needle heating block was found to function improperly. The needle should be heated to a temperature between 250 ° and 300°C; however, at no time did the temperature exceed 240°C. Despite this problem, the warning lights of the instrument did not indicate a malfunction. When this was discussed with the manufacturer, they stated that BDDIS do not claim that the BACTECsystem sterilizes the needle (S. Siddiqi, personal communication). Although this may be the BDDIS current position, their "Operation and Maintenance Manual" dearly refers to the needle heating system as a "needle sterilizer" and their field representatives commonly refer to the process as sterilization of the needle between culture bottle sampling.
34
In summary, a number of potentially serious problems exist with the BACTEC460 TB System. First, defective operation of the needle heating block can occur and can remain undetected by the system. At the present time, the only way that this function can be monitored is by the company. This should be corrected by a modification of the warning light component that will indicate when the heating block fails to reach the minimum specified temperature. Second, users of the BACTEC460 TB system should realize that cross-contamination between bottles can occur and be
Letter
separated by several uncontaminated bottles. Although this problem has been well documented, it may not be appreciated by most users. Finally, if BDDIS denies that their instrument sterilizes the sampling needle, this should be clearly stated in their product manual and further clarified by their field representatives. Moreover, if the manufacturer takes this position, then the liability of false-positive mycobacterial cultures will be shifted to the laboratory user. The legal ramifications of this should be carefully considered before a laboratory uses such a system.
REPLY
We appreciate Dr. Murray's concern about crosscontamination with the BACTECTB System. We realize that the occurrence of cross-contamination, though rare, is a serious problem, and when it occurs, it warrants immediate attention. The BACTEC 460 instrument has been in use for mycobacteriology for more than 10 years. The incidence of cross-contamination was brought to our attention only in 1987. Realizing the gravity of this problem, we began investigating solutions to these rare occurrences at that time. In the meantime, we felt an obligation to share this information with our BACTECusers. A bulletin was sent out (BACTECDATA, Nov. 1987) defining the problem--"cross-contamination occurs in the BACTEC 460 System when organisms from a truly positive vial are transferred via the needles into a subsequent negative vial or vials." In the same notification, we recommend specific preventive maintenance procedures and pointed out that "on occasion, faulty circuitry in the 460, that alters the needle heating efficiency, can contribute to the problem." To minimize further the potential for cross-contamination, a modification of the electronic boards that regulate the heating cycle was developed. This resulted in an increase in the needle heating time. Several instruments were fitted with these modified boards and subjected to rigorous testing in-house in order to assess whether cross-contamination could still occur. Having seen no evidence of cross-contamination, we were confident that this approach would take care of the problem. The new board was rigorously field-tested and was found to prevent crosscontamination effectively (Conville and Witebsky, 1987). A program of replacing old boards with the new ones began in 1989. Recently, however, some of the newly installed boards have been found to be faulty. They have not produced the necessary temperature for a sufficient duration of time. We are taking c o r -
rective measures. Presently, every new board is being checked thoroughly for temperature output before it is installed. We also plan to check all previously installed new boards. At selected sites we intend to implement a special temperature-monitoring device for a long-term study of the efficiency of the new board and needle heater. In addition, each of our field service engineers will soon have special equipment to monitor needle heater performance at the time of preventive maintenance visits. Concerning the issue of "needle heater" versus "needle sterilizer" which Dr. Murray has raised, initially, we incorrectly described the 460 needle heater as a needle sterilizer. In the early 1980s we began the modification of our operating manuals to describe the system more properly as a needle heater. At the same time, our sales representatives and field service engineers were made aware of this distinction. As early as Fall 1983, our BACTECNEWS, sent out to all BACTECusers, indicated that "needles should be considered contaminated and handled with appropriate caution at all times." "Prior to the removal of the needle set for autoclaving and cleaning, you may want to process an uninoculated vial as the final vial. This provides a heating cycle after the last patient specimen tested, which further reduces potential for accidental exposure." We do realize that malfunctions can occur in the BACTEC 460, as in any other instrument, and users should routinely monitor system performance. Falsepositivity, either due to cross-contamination or from environmental contamination, can occur in bacteriology. We have taken corrective measures with the BACTEC TB system and are continually looking into ways either to eliminate or minimize this occurrence further.
Salman H. Siddiqi, PhD Research Fellow B E C T O N D I C K I N S O N D I A G N O S T I C I N S T R U M E N T SYSTEMS
