Our results show that diluting a medicine manually is prone to deviating concentrations, even by experienced personnel. The concentrations in the group “Pharmacy” were precise and accurate. Of the manual dilution methods, the 3rd led to most accurate and precise concentrations. This means that a lower starting concentration leads to higher precision and accuracy. Contamination occurred in groups 2 and pharmacy group.
The dilution method in group 1 resulted in three cases with concentrations which could result in respiratory depression when injected intrathecally. If five millilitres of these solutions would have been injected it would result in an injected dose of 1 to 1.5 mg intrathecally. Therefore, this method should not be used. It illustrates that this dilution process is at risk of creating dangerously high morphine concentrations. The erroneous high concentration is possibly achieved because no new extraction needle was used and by the lack of rinsing off the first extracting needle in this protocol, thereby leaving a small volume of high concentration morphine in the needle. A second explanation may be because the solution did not mix properly in the 100 ml container.9
The precision of group 2 and 3 was clinically acceptable, even though the accuracy was limited. When these methods of dilution are clinically applied, one has to aim for the lower limit of the therapeutic range of intrathecal morphine to prevent an inadvertent high dose. The higher concentration is possibly caused by the excess volume of a 1.0 ml morphine ampoule, which has to be more than 1.0 ml to allow an extractable volume of 1.0 ml.12 We believe that methods 2 and 3 are inherently safer methods, because the needle is rinsed if no new needle is used and the solution is mixed by aspiration of 9 ml of saline. This is supported by the study of Benkhadra et al., which shows that mixing of the syringe results in a homogenous distribution of the solution.9 Even though this was a relatively minor effect, every cause for imprecision should be excluded.
Most remarkably, 2 groups contained a contaminated sample, despite precautions of clean preparation, such as wearing non-sterile gloves and caps and swiping the ampoule with 70% ethanol before opening. We did not instruct the participants to wear face masks, because we prepared the solutions as in daily practice. We found a high frequency of contaminations, even in experienced healthcare providers. Dilution steps did not appear to increase bacterial contamination. Given the rate of contamination, it is surprising that the incidence of bacterial meningitis after an intrathecal injection is around 1: 53.000.11 The contamination with the aerobic spores in this study is most likely not airborne so wearing a face mask during preparation probably wouldn’t have changed the results.
Several pathways for contamination of intrathecal injection are described. One pathway consists of bacteria originating from the oropharynx of the healthcare provider falling on the sterile area and instruments. A second pathway is that contaminated particles fall in the ampoule when this is opened.10, 13 The current study shows contaminated medication could be an important pathway of introducing a microorganism into the cerebrospinal fluid and our precautions fail to prevent contamination by this bacteria.14
This contamination with gram-positive rods is most likely bacillus cereus which is also part of the human skin flora and commonly associated with contamination.15 The spores of bacillus cereus are alcohol-resistant.16 In healthy patients, the possibility of Bacillus Cereus infection in the central nervous system is low because of intact host resistance. In immunocompromised patients, however, Bacillus Cereus was identified as causative microorganism for meningitis leading to fatal outcomes.17, 18 The inability to remove the spores with alcohol might pose a risk in these patients and disinfection procedures with the use of solutions containing chlorine or hydrogen peroxide should be considered
Bupivacaine was added to measure control of volume. The study showed that the aspiration of 2 ml into a 5 ml syringe is accurate. Furthermore, bupivacaine has antibacterial properties, making it of interest for the measurement of contamination.19 Despite this antibacterial effect, contamination occurred in 2% of the syringes. The difference in bupivacaine concentration between group pharmacy (2.5 mg/ml) and the other groups (2.0 mg/ml) is unlikely to affect the contamination rate.20, 21
A few recommendations can be made based on this study. Firstly, prefabricated drugs should be preferred in clinical practice. If this is not available, one should dilute from the lowest possible starting concentration and mix the syringe during the process. Secondly, sterile precautions should be undertaken when medication for intrathecal use is aspirated since bacterial contamination is likely to occur as shown by Zacher et al.13 Several hospitals routinely prepare drugs with high microbiological risk, such as intrathecal administrations, in a cleanroom environment, either centrally in the hospital pharmacy or decentral in a laminar flow cabinet in close proximity to the operation theatres. Contamination occurs during diluting of drugs and or during aspiration of drugs from an ampoule. Ready to administer (RTA) drugs could be considered as intrathecal drugs. Whether this precaution lowers contamination risk should be investigated further. Thirdly, clinical studies regarding intrathecal morphine sometimes do not describe the manufacturing process22 or dilute manually23. Manually diluted drugs could yield a variance in dose with a different effect/side effect ratio. Therefore, the manufacturing process should be described in clinical studies.
A limitation of this study is that we did not determine the species beyond the gram-stain.
Gram positive aerobe spore forming bacteria are either Bacillus Antracis or Bacillus Cereus. The first would be very unlikely. Additionally, the bupivacaine concentration in the group pharmacy differed from the other groups, although this range of bupivacaine is unlikely to affect the contamination rate of Bacillus Cereus.21 Also, the incidence of contamination was not less in the pharmacy group.