Interleukin-6 (IL-6) Unit Converter
Interleukin-6 (IL-6) Unit Converter
pg/mL and ng/L are the same number for IL-6, and ng/mL is a thousandth of either. The harder fact is that IL-6 has no established decision threshold — the same serum reads fiftyfold apart on different assays.
Interleukin-6 (IL-6) converter
pg/mL = ng/LIL-6 4.2 pg/mL, Elecsys assay
Units and conversion
1 pg/mL = 10⁻¹² g ÷ 10⁻³ L = 10⁻⁹ g/L = 1 ng/L
ng/mL = pg/mL ÷ 1000
- pg/mL and ng/L
- the same number. A picogram per millilitre is a nanogram per litre — the thousandfold step down in mass is cancelled by the thousandfold step up in volume
- ng/mL
- a thousandth of the pg/mL figure. It appears mostly in research papers and in some multiplex assay outputs; a clinical IL-6 of 4.2 pg/mL is 0.0042 ng/mL
- no molar unit
- IL-6 is a glycoprotein of about 21 to 28 kDa depending on glycosylation state, and assays are calibrated against a preparation rather than against a defined molecular species, so a pmol/L figure would be spurious
- no decision threshold
- none is published here because none is established. The package insert for the assay these reference values come from states that IL-6 values alone are not indicative of the need for intubation or ventilation, and a 2024 umbrella review of IL-6 diagnostic accuracy found published cut-offs varying with the reference standard, the sample type and the measurement method
- assay disagreement
- the practical size of the problem. Four IL-6 immunoassays run on the same samples returned mean concentrations of 1.08, 1.68, 2.07 and 53.1 pg/mL. A cut-off derived on one of those assays means nothing on another
Worked example
IL-6 4.2 pg/mL, Elecsys assay
4.2 pg/mL = 4.2 ng/L — identical units
4.2 ÷ 1000 = 0.0042 ng/mL
Within the quoted interval of up to 7 pg/mL for apparently healthy adults on that assay
A value inside the interval does not exclude an inflammatory response that has not yet developed: IL-6 peaks early and falls fast
IL-6 in each unit
| pg/mL | ng/L | ng/mL |
|---|---|---|
| 1.5 — typical limit of quantitation | 1.5 | 0.0015 |
| 4.2 | 4.2 | 0.0042 |
| 7.0 — 95th centile, healthy adults, Elecsys | 7.0 | 0.0070 |
| 100.0 | 100.0 | 0.1000 |
| 1,000.0 | 1,000.0 | 1.0000 |
| 5,000.0 — typical upper measuring limit | 5,000.0 | 5.0000 |
Where IL-6 is used, and what is and is not established
| Setting | What IL-6 does there | Is there an accepted threshold? |
|---|---|---|
| Sepsis and early bacterial infection | Rises within one to two hours, several hours ahead of CRP, and falls fast once the stimulus stops | No. Published cut-offs vary widely between studies and assays, and none has been adopted as a decision rule |
| Cytokine release syndrome after CAR-T therapy | Rises with the syndrome and falls with tocilizumab, so it is used to follow the course | No. ASTCT consensus grading is clinical — fever, hypotension, hypoxia — and does not use a cytokine concentration |
| Severe COVID-19 | A marker of inflammatory severity; the assay whose interval is quoted above was authorised in that setting | No. The package insert states that IL-6 values alone are not indicative of the need for intubation or ventilation |
| Chronic inflammation, ageing, adiposity | Modestly raised, which is why a mildly high value in a well patient is usually uninformative | No, and none is likely — the signal overlaps completely with health |
Why this page converts units and refuses to publish a cut-off
Interleukin-6 is a pleiotropic cytokine released by macrophages, endothelium, fibroblasts and adipose tissue within one to two hours of an inflammatory stimulus. It is the signal that drives hepatic synthesis of C-reactive protein, which is why it rises several hours ahead of CRP and falls again as quickly once the stimulus stops. That kinetic profile is the source of its appeal and of most of the difficulty in using it: a value is a snapshot of a fast-moving curve, and when in the illness the sample was drawn matters more than for almost any other inflammatory marker.
The units are straightforward. A picogram per millilitre is a nanogram per litre, because the thousandfold step down in mass is exactly cancelled by the thousandfold step up in volume, so pg/mL and ng/L never differ. ng/mL, which turns up in research papers and multiplex panels, is a thousandth of either. There is no molar unit: IL-6 is glycosylated to a variable extent, its apparent mass ranges from about 21 to 28 kDa, and assays are calibrated against a reference preparation rather than a defined molecular species.
What this page will not do is publish a decision threshold, because there is not one to publish. The scale of the problem is measurable: four IL-6 immunoassays run on the same samples returned mean concentrations of 1.08, 1.68, 2.07 and 53.1 pg/mL — a fiftyfold spread on identical serum. A cut-off derived on one platform therefore carries no meaning on another, and the literature reflects that, with a 2024 umbrella review of IL-6 diagnostic accuracy attributing the variation in published cut-offs to the reference standard, the sample type and the measurement method. Even the manufacturer of the assay whose reference interval is quoted above states in the package insert that IL-6 values alone are not indicative of the need for intubation or ventilation.
So the one interval this page carries — up to 7 pg/mL, the 95th percentile of 817 apparently healthy adults — is labelled with the assay it came from, and it is an expected-value range, not a decision rule. Used well, IL-6 is a trend on one platform in one patient: rising early in a deteriorating patient before CRP moves, falling as a cytokine release syndrome responds to tocilizumab. Used badly, it is a single number compared against a threshold borrowed from a study that used a different assay, and it will mislead.
Frequently asked questions
Is IL-6 in pg/mL the same as ng/L?
Yes, exactly. A picogram per millilitre is a nanogram per litre: the thousandfold reduction in the mass prefix is cancelled by the thousandfold increase in the volume. ng/mL is different — it is the pg/mL figure divided by a thousand.
What is a normal IL-6 level?
One widely used assay quotes up to 7 pg/mL as the 95th percentile of apparently healthy adults. That figure belongs to that assay: IL-6 results are not transferable between platforms, so use the range printed on your own report.
Is there an IL-6 cut-off for sepsis?
No established one. Published thresholds vary widely with the assay, the sample type and the reference standard used in each study, and no professional body has adopted one. IL-6 is best read as a trend on a single platform alongside the clinical picture.
Why do IL-6 results differ so much between laboratories?
Because the assays disagree. Four IL-6 immunoassays run on the same samples returned mean concentrations of 1.08, 1.68, 2.07 and 53.1 pg/mL. Different antibodies, different calibrators, and no full harmonisation between manufacturers.
Does IL-6 replace CRP or procalcitonin?
No. IL-6 rises earlier than CRP because it is the signal that induces it, and it falls faster, which can be useful in the first hours. But it is less specific than procalcitonin for bacterial infection, has no accepted threshold, and is not comparable between assays.
Related calculators
References
- Roche Diagnostics. Elecsys IL-6. Package insert, 2020. (Reference range up to 7 pg/mL, 95th percentile, n = 817; measuring range 1.5–5,000 pg/mL.)
- Thompson DK, Huffman KM, Kraus WE, Kraus VB. Critical appraisal of four IL-6 immunoassays. PLoS One. 2012;7(2):e30659.
- Lee DW, Santomasso BD, Locke FL, et al. ASTCT consensus grading for cytokine release syndrome and neurologic toxicity associated with immune effector cells. Biol Blood Marrow Transplant. 2019;25(4):625–638.
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.