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Is There an Acceptable Level of Nontuberculous Mycobacterium in a Heater-cooler Water Tank?

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      Background

      Heater-cooler units (HCUs) are frequently used in cardiac surgery and have been implicated in nontuberculous Mycobacterium (NTM) infections. A patient developed a NTM infection after undergoing cardiac surgery and the HCU was evaluated for an epidemiologic link. This study assesses challenges of interpreting significance of low numbers of colony forming units (CFU) of NTM in the water tank of a HCU.

      Methods

      The HCU was removed from service. A 220 milliliter (ml) water sample was collected from the tank drain of the HCU after disinfection and sent to an outside laboratory to evaluate for NTM. NTM was identified in the initial specimen. The HCU was re-disinfected and re-cultured. Repeat cultures grew the same NTM as the first sample. Disinfection practices were reviewed and all HCU disinfection aligned with manufacturer's instructions for use. With continued growth of NTM, the HCU was sent to the manufacturer and internal water paths were replaced. Upon return of the HCU to the hospital, it was disinfected and cultured.

      Results

      Initial water sample grew 14 CFU/ml of Mycobacterium immunogenum. This species did not match the patient's clinical isolate. Repeat culture after disinfection grew 36 CFU/ml of the same organism. Water cultures obtained after the manufacturer replaced the HCU's internal water paths grew 14 CFU/ml of the same NTM. After review of Centers for Disease Control and Prevention (CDC) guidance regarding acceptable levels of colony counts in dialysis and endoscopes, in addition to no epidemiological link of this NTM species to any patient infections, the HCU was returned to service.

      Conclusions

      Currently, there is no standardized threshold for NTM in HCUs or in municipal water supply. The use of filters may not eliminate all NTM in tap water. A standardized threshold needs to be set for allowable limits of recovered NTM in HCUs in order to return to service safely.
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