No funding

Patients will be invited to participate from an outpatient nephrology clinic at Hammersmith Hospital. They will have a microdialysis catheter inserted into a peripheral vein and connected to a sampling system. The samples will be sent for analysis in the hospital laboratory. All medical devices to be used are already CE-marked and approved for these purposes.

Patients attending Professor Tam's outpatient clinic will be invited to participate in the study. After time to read the relevant protocol and consent form, and agreeing to proceed, a patient will be taken to a separate room in the outpatient department by Dr. Robert M. Learney.

In this room, the patient will have a suitable vein identified on the forearm or back of the hand. Dr. Learney will then insert a venous cannula into the vein with aseptic technique, secure it in place, and then draw a 3-5mL preliminary blood sample for creatinine and potassium analysis in the hospital lab. Outpatients attending the nephrology department would have one blood test for these biochemicals (and others) taken as part of routine care anyway. This sampling would replace the routine blood test for this attendance.

Dr. Learney will then insert a single-use type 67 intravenous microdialysis catheter into the venous cannula, attach a sampling vial to the outlet of the catheter, and a pump for delivering a sterile saline solution to the inlet of the catheter. All devices and liquids are CE-marked and licensed for human use. Half of the 20 patients in the study will have a short 1cm microdialysis catheter inserted, and half will have a longer 2cm microdialysis catheter inserted.

The pump will be started at a flow rate of 2 microlitres per minute, and 50 microlitres of sample will be collected over the next 25 minutes. The patient will be free to walk around and/or read a book, but must return at the end of 30 minutes for the sample vial to be exchanged for a new one. At this point the pump flow rate will be adjusted down to 1 microlitre per minute by turning a small dial on the side. The patient will be free again for the next 50 minutes to produce the next 50 microlitres of sample. The sample vial will be exchanged again at this point, and the flow rate adjusted down one final time to 0.5 microlitres per minute for 100 minutes to collect a final 50 microlitres of sample.

After this final experiment, the patient will have the microdialysis catheter removed from the cannula for inspection and disposal. A final 3-5mL blood sample will be withdrawn from the cannula, and if this is not possible then it will be taken from a different visible vein. These first and last blood draws ensure that the baseline levels of the two chemicals of interest have not changed significantly during the three hours of the experiment.

All samples collected in this experiment will be assayed in the hospital laboratory for the biochemicals of interest.

Inclusion Criteria:

• Attending nephrology outpatient clinic
• Aged 18-85
• Able to spare 3.5 hours to participate in study

Exclusion Criteria:

• Lacks capacity to consent
• Unable to consent in English
• No visible peripheral veins

Normal cross-section of adults attending outpatient nephrology clinic