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Are trough tobramycin concentrations taken from a central venous catheter accurate enough to safely use?
  1. Hannah Tumelty1,
  2. Geoff Shenton2,
  3. Malcolm Brodlie3,4
  1. 1 General Paediatrics, Great North Children's Hospital, Newcastle upon Tyne, UK
  2. 2 Paediatric Oncology and Haematology, Great North Children's Hospital, Newcastle upon Tyne, UK
  3. 3 Translational and Clinical Research Institute, Newcastle University, Newcastle upon Tyne, UK
  4. 4 Paediatric Respiratory Medicine, Great North Children's Hospital, Newcastle Upon Tyne, UK
  1. Correspondence to Dr Hannah Tumelty, Great North Children's Hospital, Newcastle Upon Tyne NE1 4LP, UK; hannah.tumelty{at}

Statistics from


A 15-year-old girl with cystic fibrosis (CF) and needle phobia has recently had a port-a-cath placed to facilitate regular intravenous antibiotic administration. She is admitted to the respiratory ward to receive ceftazidime and tobramycin to treat Pseudomonas aeruginosa. Local guidance requests a blood sample for trough tobramycin concentration to be taken peripherally prior to the second dose. This is due to concerns about the risk of falsely elevated concentrations if sampled from the same line used to give the drug. The patient is anxious and asks, “why can’t you just take it from my port like other blood tests?”

In other departments, such as paediatric oncology, antibiotic concentrations are sampled from the same central line through which the drug is given, prompting us to question could we also spare our respiratory patients the trauma of a peripheral venepuncture to measure tobramycin trough concentrations?

Structured clinical question

In children with CF (patient), do trough tobramycin concentrations taken from the central venous catheter through which the drug was administered (intervention) differ in a clinically significant way (outcome) from those taken peripherally (comparison)?


Secondary sources

The Cochrane Library was searched using a combination of the terms as outlined below for primary sources and no suitable papers were found.

Primary sources

We searched PubMed using the search terms ‘tobramycin level’ AND venous OR peripheral OR trough OR comparison.

After reviewing the abstracts and discarding duplicates we were left with seven papers, of which we were able to obtain the full text for six. All papers that involved a comparison of tobramycin concentrations were included. The citations of the chosen papers were also reviewed to ensure that all relevant papers were included.


See table 1.

Table 1

Studies comparing antibiotic concentrations from central and peripheral samples


All papers identified were prospective trials where children had paired central and peripheral blood samples taken and thus acted as their own controls. Five out of six studies involved children with CF. Studies largely involved peripherally inserted central catheter lines or port-a-caths (totally implantable venous access devices).

Tobramycin dosing schedules varied between studies (either two or three times per day, rather than the more frequent current practice of once daily and is the policy at our department). Once daily regimens mean that trough concentrations would be taken approximately 23 hours after the line was last used for tobramycin administration.

A consistent finding across studies was that peak concentrations taken from central venous access are often higher than peripheral concentrations, but this does not necessarily require a change in antibiotic dose. Trough concentrations, which we are specifically interested in, appear to have better correlation between peripheral and centrally accessed samples. Throughout all the studies the situation did not occur where a trough concentration taken from a central venous catheter would have caused an unnecessary reduction in antibiotic dose.

All papers agree that a standardised method of sampling from central venous catheters that involves a good volume of flush and discard is essential. Consideration should also be given to the possibility of ‘diluted samples’, where insufficient discard is taken and the sample becomes contaminated with the saline used to flush the device.1

In conclusion, the evidence available to us suggests that using a tobramycin trough concentration taken from a central venous access used to give the drug would not lead to inaccurate changes in dosing. A prospective study comparing peripheral and central access samples of once daily tobramycin dosing in children with CF has not been performed, however.

Clinical bottom line

  • Studies that have compared tobramycin trough concentrations sampled peripherally versus from the same central venous access used to give the drug have found that central access sampling does not lead to inaccurate changes in dosing (grade C).

  • A standard operating procedure where appropriate volumes of flush and discard are used is important (grade C).

  • No prospective studies have been performed in children with cystic fibrosis prescribed once daily intravenous tobramycin (as is common current clinical practice in cystic fibrosis care) (grade C).

Ethics statements

Patient consent for publication



  • Contributors HT contributed to the conception of the work and wrote the first draft of the paper. GS and MB contributed to the conception of the work and commented on the final draft of the paper.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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