Written and maintained by CASRAI Editorial Board
Last updated
General reference for researchers and lab staff. It is not medical advice and contains no procedure instructions.
Short answer
Microdialysis is a sampling technique in which a thin probe with a semipermeable membrane is perfused with fluid at a very low flow rate. Small, freely diffusible molecules in the surrounding tissue cross the membrane into the perfusate, which is collected as dialysate and analysed. It measures the unbound, extracellular fraction of an analyte, and the result is only as quantitative as the recovery calibration behind it.
Definition and scope
A microdialysis probe (or catheter) has a tip built from two concentric tubes, and the outer wall is a semipermeable membrane. A pump drives a perfusion fluid through the probe at low flow; molecules diffuse across the membrane in both directions, and the perfusate returns through the central tube to a collection vial (PMC3663257). The approach gives continuous sampling of the protein-unbound, extracellular concentration of a substance (PMC9633485), which is why it is used for tissue pharmacokinetics and for endogenous markers in tissues such as brain and skin.
Two points set the scope. First, only molecules small enough for the membrane pass, so what you measure depends on the membrane cut-off. Second, the technique is invasive; in the clinical cerebral literature it is described as usable only in patients who already require neurocritical care or neurosurgery (PMC3663257). Methods such as biological assays and chromatography-based quantitation sit downstream of the dialysate, not in the probe itself.
How it is used in practice: the sampling set-up
The published papers describe the same building blocks.
- Probe and membrane. A dermal catheter in one study was a thin hollow polyarylethersulfone (PAES) fibre, and 100 kDa cut-off membranes approved for clinical use widen the range of detectable analytes to cytokines (PMC5978059). A custom-made research probe used polysulfone with a 30 kDa cut-off and a 2 mm exchange length for mouse and rat brain (PMC4545111).
- Perfusate. A tissue-compatible buffer. The cerebral clinical study used a solution of 147 mM NaCl, 2.7 mM KCl, 1.2 mM CaCl2 and 0.85 mM MgCl2 (PMC3663257).
- Flow rate. Typically 0.3 μL/min in that cerebral work (PMC3663257) and typically 1-2 μL/min in a voriconazole tissue study (PMC9633485).
- Collection and analysis. Dialysate vials are analysed off-line, for example by quantitative mass spectrometry.
Why recovery is the central problem
Perfusate moves too fast to reach full equilibrium with the tissue, so the dialysate concentration is lower than the concentration in the tissue fluid. Two terms matter (PMC5978059):
- Absolute recovery: the total amount of analyte extracted per unit time.
- Relative recovery: dialysate concentration divided by the concentration in the fluid around the probe.
If absolute concentrations are required, careful calibration for relative recovery (extraction efficiency) is needed (PMC3663257). In vivo the true concentration is mostly unknown, which is why in vivo recovery has to be estimated indirectly (PMC5978059). It is the same idea as any calibration: a measured signal is only interpretable once its relationship to the true value is established.
Recovery depends on the analyte and the probe
The figures below come from different probes and set-ups, so they show that recovery varies widely; they are not interchangeable.
| Analyte (study) | Probe or set-up | Reported recovery | Source |
|---|---|---|---|
| Glucose, lactate, acetylcholine, ATP | Custom polysulfone probe, 30 kDa cut-off, in vitro | About 20% | PMC4545111 |
| Neuropeptides of 1,000-1,600 Da | Same probe, in vitro | 2-6% | PMC4545111 |
| Compounds up to 500 Da | Same probe | Little affected by size | PMC4545111 |
| Voriconazole and its N-oxide metabolite | Flow 1-2 μL/min, in vitro | Median 87.6% and 91.1% | PMC9633485 |
In the voriconazole study, the study day was the main source of recovery variability (46.3%), while the individual catheter contributed 4.33% (PMC9633485). A practical reading: do not assume one calibration carries over between sessions.
Calibration methods compared
| Method | What is done | What the sources say about fit | Source |
|---|---|---|---|
| Retrodialysis by drug | A known drug concentration is added to the perfusate and compared with what is recovered | Used mainly for in vivo recovery; unsuitable for determining relative recovery of endogenous molecules; found feasible as calibration for voriconazole | PMC3663257, PMC5978059, PMC9633485 |
| No-net-flux | The concentration in the perfusate is varied | Used for in vivo recovery | PMC3663257 |
| Retrodialysis (general) | Used to correct for incomplete equilibrium | Applies to diffusible (unbound) substances | PMC4051625 |
Where it matters for labs and clinics
Because microdialysis reports the unbound extracellular fraction, it answers a different question from a plasma or total-tissue assay. Plasma levels are often poor predictors of activity in the central nervous system, and cerebrospinal fluid is a different compartment from brain extracellular fluid (PMC3663257). An AAPS-FDA workshop white paper on the technique exists (Chaurasia et al., J Clin Pharmacol 2007;47:589-603, doi 10.1177/0091270006299091).
Reporting checklist drawn from these papers
This editorial checklist lists the parameters the cited papers themselves vary or report, so another lab can interpret the result:
- Membrane material, molecular-weight cut-off and exchange length.
- Perfusate composition and flow rate.
- Recovery method (retrodialysis, no-net-flux or in vitro) and when it was determined, given that day-to-day variability was the largest component in one study.
- Whether the reported concentration is raw dialysate or recovery-corrected.
Record these in the methods section and in the laboratory SOP. For analytical limits on the dialysate assay, see setting the LLOQ for a bioanalytical assay.
Common misunderstandings
- Dialysate is not tissue concentration. Absolute quantification needs a recovery calibration (PMC3663257).
- Retrodialysis does not calibrate endogenous molecules. It is unsuitable for relative recovery of endogenous compounds (PMC3663257).
- Larger molecules do not recover like small ones. Neuropeptides recovered at 2-6% against about 20% for small metabolites in one probe (PMC4545111).
- It does not sample protein-bound drug. It samples the unbound, diffusible fraction (PMC9633485, PMC4051625).
FAQ
What is microdialysis used for?
Continuous sampling of unbound, extracellular analytes in tissue, for example drug concentrations in tissue pharmacokinetics and cytokines in skin (PMC9633485, PMC5978059).
What does a microdialysis probe consist of?
Two concentric tubes with a semipermeable outer membrane at the tip; perfusate enters, exchanges with the tissue fluid, and returns to a vial (PMC3663257).
How is recovery calibrated?
Mainly by retrodialysis (for drugs) or no-net-flux; see the table above.
Sources
All read as NCBI full text on 2026-11-08.
- PMC3663257, Cerebral microdialysis in clinical studies of drugs: pharmacokinetic applications, J Pharmacokinet Pharmacodyn. ncbi.nlm.nih.gov/pmc/articles/PMC3663257
- PMC5978059, Cytokine and Chemokine Recovery Is Increased by Colloid Perfusates during Dermal Microdialysis, Materials. ncbi.nlm.nih.gov/pmc/articles/PMC5978059
- PMC9633485, Microdialysis of drug and drug metabolite (voriconazole), Pharm Res. ncbi.nlm.nih.gov/pmc/articles/PMC9633485
- PMC4545111, Custom-made Microdialysis Probe Design, JoVE 2015. ncbi.nlm.nih.gov/pmc/articles/PMC4545111
- PMC4051625, Clinical Pharmacology review of microdialysis. ncbi.nlm.nih.gov/pmc/articles/PMC4051625
