Few numbers in gene therapy cause as much confusion as AAV titer. A single vector lot can be described as 1×1013 capsids/mL, 3×1012 vg/mL, and 3×1010 TU/mL — all true, all different, because each unit counts a different population of particles. This guide explains what vg/mL, GC/mL, capsids/mL and TU/mL each mean, how they relate, how each is measured, and how to turn a titer into a patient dose — so the right number goes on the certificate of analysis.
The four AAV titer units
Recombinant AAV is produced as a heterogeneous mix of particles, and the titer you quote depends on which subset you count:
- vg/mL (vector genomes per mL) — particles carrying a packaged vector genome. The clinical dosing unit.
- GC/mL (genome copies per mL) — the genome-copy count returned by PCR; used interchangeably with vg/mL for a purified lot.
- capsids/mL (cp/mL, total particles) — every assembled capsid, whether full, partially filled, or empty.
- TU/mL or IU/mL (transducing / infectious units) — particles that actually infect a cell and express the transgene in an assay.
The trap is treating these as interchangeable. They are not: they form a strict hierarchy, and the gaps between them are quality attributes in their own right.
How the units nest and convert
Every transducing unit must carry a genome, and every genome sits inside a capsid — so the populations nest. Total capsids is the largest number, vector genomes a subset of it (the full capsids), and transducing units a smaller subset still (the genome-containing particles that are also infectious).
Two ratios bridge the three numbers. The full:empty ratio links capsids to genomes (full % = vg ÷ capsids), and the particle-to-infectivity ratio links genomes to transducing units (often 50:1 to >1000:1, where lower means a more potent vector). Convert in either direction once you have measured one ratio.
Convert between AAV titer units instantly
vg/mL, GC/mL, capsids/mL and TU/mL with full:empty correction, plus a vg/kg dose calculator.
Full vs empty capsids
Empty capsids are the reason capsid titer and genome titer diverge. AAV packaging is inefficient, so a crude harvest can be 50–90 % empty; anion-exchange polishing enriches the full fraction, and a good drug substance is often >70 % full. Empties carry no therapeutic genome but still present antigen to the immune system and still count against the total capsid dose, so the full:empty ratio is a release-defining quality attribute, not a cosmetic one.
This is also why dosing on capsid titer alone is unsafe: two lots at the same capsids/mL can deliver very different genome doses if their full fractions differ. The genome titer (vg/mL) is what determines efficacy; the full:empty ratio tells you how much empty capsid the patient receives alongside it.
How each titer is measured
Each unit has its own assay, which is part of why the numbers differ. No single method gives all three.
| Unit | Counts | Primary methods |
|---|---|---|
| vg/mL = GC/mL | Packaged genomes | qPCR, droplet digital PCR (ddPCR) |
| capsids/mL | All particles (full + empty) | Capsid ELISA, SEC-MALS, AUC, OD260/280 |
| Full:empty ratio | Full fraction of capsids | AUC, charge-detection MS, cryo-EM, AEX-HPLC, SEC-MALS |
| TU/mL = IU/mL | Infectious particles | Cell-based transduction + qPCR or expression readout |
For genome titer, ddPCR has become the reference choice over qPCR: it counts genome copies absolutely, without a standard curve, and is more reproducible across operators and labs. Capsid and full:empty methods range from a quick capsid ELISA to high-resolution analytical ultracentrifugation; the deeper characterisation of the genome itself, and the qPCR/ddPCR chemistry behind it, follow the same logic as general nucleic acid quantification.
Why the same lot gives different titers
Beyond the unit you choose, the method matters enormously. A landmark AAV2 reference-standard study coordinated across many laboratories found vector-genome titers spread over roughly an order of magnitude, and infectious titers over about three orders of magnitude, for aliquots of the same material. The drivers were the qPCR target (ITR versus transgene), the standard curve, DNA extraction and linearisation, and the infectivity assay design.
The practical takeaways: never compare titers across methods without a bridging study; quote the method alongside the number; and where possible calibrate against a reference standard. The same discipline applies when comparing your in-house process titer to a published AAV production yield — a 3× difference may be assay, not process.
From titer to dose volume
Clinical AAV dosing is specified in vg/kg, so the volume to administer comes straight from the genome titer:
total dose (vg) = dose (vg/kg) × body weight (kg)
volume (mL) = total dose (vg) ÷ titer (vg/mL)
Worked example: dose volume and empty-capsid load
A systemic dose of 2×1013 vg/kg for a 5 kg patient, from a lot at 2×1013 vg/mL that is 30 % full.
total dose = 2×1013 × 5 = 1×1014 vg
volume = 1×1014 ÷ 2×1013 = 5 mL
capsids delivered = 1×1014 ÷ 0.30 = 3.3×1014 capsids (2.3×1014 empty)
The patient receives 5 mL carrying 1×1014 genome-containing vectors — but also 2.3×1014 empty capsids. Enriching to 70 % full would cut that empty load roughly five-fold for the same genome dose. The AAV titer converter runs this dose and full:empty arithmetic directly.
Figure 2. One lot, three units (log scale): capsids > vector genomes > transducing units, spanning orders of magnitude.
Frequently Asked Questions
What is the difference between vg/mL and GC/mL for AAV?
Both count packaged AAV genomes per millilitre and are used interchangeably for a purified lot, both measured by qPCR or ddPCR. Strictly, GC/mL is the raw genome-copy count from PCR, while vg/mL implies those copies are inside intact vectors.
What is the difference between capsid titer and genome titer?
Capsid titer (capsids/mL) counts every particle, full or empty; genome titer (vg/mL) counts only particles with a vector genome. Capsid titer is always higher, and the ratio is the full:empty ratio (full % = vg ÷ capsids).
How do you convert vg/mL to capsids/mL?
Divide the genome titer by the full fraction. A lot at 3×1012 vg/mL that is 30 % full is 3×1012 / 0.30 = 1×1013 capsids/mL. You need a measured capsid titer or full:empty ratio (ELISA, SEC-MALS, AUC, CDMS) to convert.
Why does the same AAV lot give different titers?
Different units count different particles, and even one unit varies with method. A coordinated AAV2 reference-standard study found vector-genome titers varied ~10-fold and infectious titers ~1000-fold across labs, depending on the qPCR target, standard, and assay. Specify dose in vg/mL against a defined method.
How do you calculate an AAV dose volume from titer?
Multiply the vg/kg dose by body weight for total vg, then divide by the vg/mL titer. A 2×1013 vg/kg dose in a 5 kg patient is 1×1014 vg; at 2×1013 vg/mL that is 5 mL.