HCV RNA IU to Copies Converter
HCV RNA IU to Copies Converter
Convert HCV RNA from IU/mL to copies/mL using the factor published for the assay that produced it — and see why there is no universal one.
HCV RNA IU/mL → copies/mL
IU/mL + named assay → copies/mL850,000 IU/mL, measured on a COBAS Amplicor HCV Monitor v2.0 (2.7 copies per IU)
The conversion, and what it is not
- IU/mL
- international units per mL, calibrated to the WHO standard — the unit all modern assays and all guidelines use
- copies/mL
- legacy unit; how many genome copies a given manufacturer’s assay counted per international unit
- factor
- published per platform, 0.9 to 5.2 copies per IU. There is no universal value
- log₁₀ offset
- log₁₀ of the factor — constant, which is why a log change is unit-independent
- 2-log fall
- a hundredfold reduction; the early virological response threshold in the interferon era
Worked example
850,000 IU/mL, measured on a COBAS Amplicor HCV Monitor v2.0 (2.7 copies per IU)
850,000 × 2.7 = 2,295,000 copies/mL
log₁₀ 850,000 = 5.93 log₁₀ IU/mL
log₁₀ 2,295,000 = 6.36 log₁₀ copies/mL
The difference, 0.43 log₁₀, is log₁₀ 2.7 — a constant offset, identical at every viral load
On a Versant bDNA assay the same 850,000 IU/mL would read 4,420,000 copies/mL — 1.9 times higher, with no change in the patient
Published conversion factors, by platform
| Assay | Manufacturer | Method | Copies per IU |
|---|---|---|---|
| Amplicor HCV Monitor | Roche | Manual RT-PCR | 0.9 |
| COBAS Amplicor HCV Monitor v2.0 | Roche | Semi-automated RT-PCR | 2.7 |
| SuperQuant | National Genetics Institute | Semi-automated RT-PCR | 3.4 |
| LCx HCV RNA Quantitative | Abbott | Semi-automated RT-PCR | 3.8 |
| Versant HCV RNA 3.0 (bDNA) | Siemens | Signal amplification | 5.2 |
| COBAS TaqMan HCV | Roche | Real-time PCR | Not specified |
| Abbott RealTime HCV | Abbott | Real-time PCR | Not specified |
Even one platform’s factor is disputed
| Source | Amplicor HCV Monitor | COBAS Amplicor HCV Monitor |
|---|---|---|
| AASLD guideline (Ghany 2009, Table 5) | 0.9 copies per IU | 2.7 copies per IU |
| Manufacturer’s calibration paper (Lee 2000) | ≈0.93 copies per IU | ≈3.1 copies per IU |
Why the log scale is the safe one
| Quantity | Depends on the conversion factor? | Why |
|---|---|---|
| Absolute value in copies/mL | Yes — up to sixfold | The factor multiplies the value |
| log₁₀ of the absolute value | Yes, but only by a constant | log₁₀ (IU × f) = log₁₀ IU + log₁₀ f |
| log₁₀ change between two results | No | The constant offset cancels when you subtract |
| A 2-log fall | No | A hundredfold reduction is a hundredfold reduction in any unit |
There is no universal factor, and that is the whole answer
The honest answer to “how many copies per millilitre is 850,000 IU/mL of hepatitis C RNA” is: it depends entirely on which machine measured it, and if that machine is a modern one, nobody has published a number. This is not pedantry about units. The reason is stated in the primary literature: “the definition of an HCV RNA copy has not been officially characterized, and as such, the conversion from HCV RNA international units will vary from assay to assay depending on how the manufacturer of a given test defines an HCV RNA copy and the corresponding relationship to international units of HCV RNA”. A copy was never a standardised quantity; an international unit is.
The published factors bear that out. AASLD’s guideline table gives 0.9 copies per IU for the manual Amplicor assay, 2.7 for the semi-automated COBAS Amplicor v2.0, 3.4 for SuperQuant, 3.8 for the Abbott LCx and 5.2 for the Versant bDNA assay — a spread of nearly sixfold, with two of the values belonging to assays from the same manufacturer. The same 850,000 IU/mL result reads as 765,000 copies/mL on one platform and 4,420,000 on another. Worse, the two real-time PCR assays that superseded all of them, COBAS TaqMan and Abbott RealTime, have no published factor in that table at all, and this page refuses to produce a figure for them rather than quietly reaching for 2.7. Even where a factor exists it may be disputed: the manufacturer’s own calibration paper puts the COBAS Amplicor relationship at about 3.1 copies per IU where the guideline table says 2.7.
Which is why results are reported and trended on the log₁₀ scale. Because the conversion is a multiplication, it becomes an addition in log space — log₁₀ (IU × factor) = log₁₀ IU + log₁₀ factor — so the offset is the same at every viral load, and when you subtract one result from another to get a change, the offset cancels. A 2-log₁₀ fall, a hundredfold reduction, is a 2-log fall whatever unit either number was reported in, provided both came from the same assay. That property is the reason the log change survived every generation of assay while the absolute copies/mL figure did not, and it is worked through on the viral load log change page.
So the practical advice is the unglamorous one: stay in IU/mL. It is the standardised unit, it is what every guideline threshold and every trial endpoint is written in, and it is what the assay actually reports. Convert to copies/mL only to make sense of an old result that was reported that way, and only when you know which assay produced it. Never compare a copies/mL result from one laboratory with an IU/mL result from another — the discrepancy can be several-fold and is easily mistaken for a real change in viral replication. The HBV DNA converter makes the same point for hepatitis B, where the factors happen to cluster more tightly. This supports a clinician’s judgement rather than replacing it. It is arithmetic on the figures entered, and it knows nothing about the patient in front of you.
Frequently asked questions
What is the conversion factor from IU/mL to copies/mL for HCV RNA?
There is no universal factor. Published values range from 0.9 copies per IU for the manual Amplicor assay to 5.2 for the Versant bDNA assay, and the real-time PCR assays that replaced them have no published factor at all. The factor belongs to the platform, not to the virus.
Why is there no single conversion factor?
Because a “copy” was never standardised. As Konnick et al put it, “the definition of an HCV RNA copy has not been officially characterized”, so each manufacturer’s relationship between copies and international units is its own. The international unit, calibrated to the WHO standard, is the unit that is comparable between assays.
Should I report HCV RNA in IU/mL or copies/mL?
IU/mL. It is the standardised unit, it is what modern assays report, and every treatment threshold and trial endpoint is written in it. Copies/mL is legacy and is only worth converting into to make sense of a historical result whose assay you know.
Does the unit affect a log change?
No, provided both results came from the same assay. The conversion multiplies the value, which in log space is a constant addition, and a constant cancels when one log value is subtracted from another. A 2-log fall — a hundredfold reduction — is the same in either unit.
What is a clinically meaningful change in HCV RNA?
A 2-log₁₀ fall, a hundredfold reduction, was the early virological response threshold used with interferon-based regimens to predict eventual cure. It has been largely superseded by direct-acting antivirals, where the endpoint is undetectable RNA rather than a proportional fall.
Related calculators
References
- Ghany MG, Strader DB, Thomas DL, Seeff LB. Diagnosis, management, and treatment of hepatitis C: an update. Hepatology. 2009;49(4):1335–74.
- Konnick EQ, Erali M, Ashwood ER, Hillyard DR. Performance characteristics of the COBAS Amplicor hepatitis C virus (HCV) Monitor, version 2.0, international unit assay and the National Genetics Institute HCV SuperQuant assay. J Clin Microbiol. 2002;40(3):768–73.
- Lee SC, Antony A, Lee N, et al. Improved version 2.0 qualitative and quantitative AMPLICOR reverse transcription-PCR tests for hepatitis C virus RNA: calibration to international units, enhanced genotype reactivity, and performance characteristics. J Clin Microbiol. 2000;38(11):4171–9.
- Pawlotsky JM, Bouvier-Alias M, Hézode C, Darthuy F, Rémiré J, Dhumeaux D. Standardization of hepatitis C virus RNA quantification. Hepatology. 2000;32(3):654–9.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.