Tacrolimus Unit Converter
Tacrolimus Unit Converter
Convert whole-blood tacrolimus trough levels between ng/mL and nmol/L, with target ranges by time since transplant and the CYP3A4 interactions that move levels fastest.
Tacrolimus converter
Mass ⇄ molarTacrolimus trough 7.5 ng/mL
Formula and conversion factor
ng/mL = nmol/L ÷ 1.2438
- 1.2438
- derived from the molecular weight of tacrolimus, 804.02 Da (1000 ÷ 804.02)
- µg/L
- numerically identical to ng/mL
- whole blood
- the conversion assumes a whole-blood specimen — a plasma or serum result cannot be compared against these ranges
Worked example
Tacrolimus trough 7.5 ng/mL
7.5 × 1.2438 = 9.3 nmol/L
= 7.5 µg/L
Typical trough targets by time since transplant
| Phase | Target C0 (ng/mL) |
|---|---|
| 0 – 4 weeks post-transplant | 10 – 15 |
| 1 – 3 months | 8 – 12 |
| Maintenance, kidney (> 12 months) | 5 – 10 |
| Maintenance, liver | 5 – 8 |
CYP3A4 interactions
| Direction | Examples |
|---|---|
| Raise levels | Azole antifungals (fluconazole, voriconazole, itraconazole), macrolides (clarithromycin, erythromycin), diltiazem, grapefruit |
| Lower levels | Rifampicin, phenytoin, carbamazepine, St John’s wort |
Why the specimen and the timing both matter
Tacrolimus is measured in whole blood, not serum or plasma. The drug partitions heavily into red blood cells — the great majority of circulating tacrolimus is intracellular — so a sample separated into serum or plasma has already lost most of the drug it is meant to be measuring. A plasma result reported by mistake is not simply inaccurate, it is meaningless, and the specimen must be a whole-blood EDTA tube run on an assay validated against blood, never a gel-separator tube intended for serum chemistry.
Monitoring is almost always a trough (C0) level, drawn immediately before the next dose, when the concentration is most stable and correlates best with total exposure. A level drawn even an hour early or late can look like a dosing trend that is really just poor timing, so consistent trough timing matters almost as much as the number itself.
Tacrolimus has a narrow therapeutic index in both directions. Under-exposure risks acute or chronic rejection of the graft; over-exposure causes nephrotoxicity, tremor, hypertension and post-transplant diabetes, and the toxic and therapeutic ranges overlap enough that a level is never read in isolation from graft function and clinical assessment. Targets are not fixed — they are highest in the first weeks after transplantation, fall progressively over the first year, and differ by the organ transplanted and by the rest of the immunosuppressive regimen.
The dominant everyday problem in practice is drug interaction through hepatic and intestinal CYP3A4, the enzyme responsible for clearing tacrolimus. Azole antifungals, macrolide antibiotics, diltiazem and grapefruit all inhibit CYP3A4 and can raise levels sharply within days; rifampicin, phenytoin, carbamazepine and St John’s wort induce it and can drop levels into a sub-therapeutic range just as quickly. Any new interacting medication, prescribed or over-the-counter, should prompt an early repeat level rather than waiting for the next routine draw.
Frequently asked questions
Why must tacrolimus be measured in whole blood?
Tacrolimus partitions heavily into red blood cells, so a serum or plasma sample has already lost most of the drug it should be measuring. Results are only valid from a whole-blood EDTA specimen.
How do I convert tacrolimus from ng/mL to nmol/L?
Multiply by 1.2438. A trough of 7.5 ng/mL is 9.3 nmol/L. The factor comes from tacrolimus’s molecular weight of 804.02 Da.
What is a normal tacrolimus trough level?
It depends heavily on time since transplant and organ. A typical kidney-transplant maintenance target is 5 – 10 ng/mL, but early post-transplant targets run considerably higher — confirm against the local protocol.
Which drugs raise tacrolimus levels?
CYP3A4 inhibitors are the main culprits — azole antifungals, macrolide antibiotics, diltiazem and grapefruit. Enzyme inducers such as rifampicin, phenytoin, carbamazepine and St John’s wort do the opposite and can drop levels sharply.
Related calculators
References
- Brunet M et al. Therapeutic drug monitoring of tacrolimus-personalized therapy: second consensus report. Ther Drug Monit. 2019;41(3):261–307.
- Staatz CE, Tett SE. Clinical pharmacokinetics and pharmacodynamics of tacrolimus in solid organ transplantation. Clin Pharmacokinet. 2004;43(10):623–53.
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.