Continuous Infusion Css/MIC Ratio Calculator
Continuous Infusion Css/MIC Ratio Calculator
Under a continuous infusion fT>MIC is 100% by construction, so the target stops being a time fraction and becomes a concentration multiple. This gives the free steady-state concentration as a multiple of the MIC, against the 4-to-8 window.
Free Css/MIC under continuous infusion
Daily dose + clearance → Css/MIC16,000 mg/day, clearance 14 L/h, MIC 16 mg/L, 30% protein bound
Formula, and why only clearance matters
- Css
- the steady-state concentration under a constant infusion: the infusion rate divided by the clearance. Rate in mg/h divided by clearance in L/h gives mg/L
- why volume of distribution does not appear
- at steady state the rate in equals the rate out, and the rate out is clearance times concentration. The volume of distribution decides how FAST steady state arrives — four to five half-lives — and the size of the loading dose, but not where the plateau sits
- fu, the unbound fraction
- (100 − binding%) / 100. The target is a free concentration, so this multiplication is not optional. For piperacillin at about 30% bound it costs 30% of the ratio; for a 95%-bound agent it costs 95%
- the inverse
- the daily dose needed for a ratio r is r × MIC / fu × CL × 24. It scales linearly with the MIC, so one doubling dilution doubles the dose required, and the answer quite often exceeds any licensed dose
- what it does not model
- stability in the infusion bag; the loading dose, without which steady state takes four to five half-lives to arrive; and the upper limit — the TARGET trial, as Berry and Kuti report it, found mortality higher below 32 mg/L and above 96 mg/L
Worked example
16,000 mg/day, clearance 14 L/h, MIC 16 mg/L, 30% protein bound
Infusion rate = 16,000 / 24 = 666.7 mg/h
Css = 666.7 / 14 = 47.62 mg/L total
Free Css = 47.62 × 0.70 = 33.33 mg/L
33.33 / 16 = 2.08 — below the 4-to-8 window the SFPT/SFAR recommendation suggests
The daily dose that would reach a ratio of 4 is 4 × 16 / 0.70 × 14 × 24 = 30,720 mg, and for a ratio of 8 it is 61,440 mg. Neither is a licensed piperacillin dose, which is the useful part of the answer: the arithmetic has run out
The highest MIC at which this dose reaches a ratio of 4 is 33.33 / 4 = 8.33 mg/L, between the 8 and 16 mg/L dilutions
Halve the clearance to 7 L/h, as renal impairment would, and the ratio doubles to 4.17 on the same dose. Double it to 28 L/h, as augmented renal clearance can, and it falls to 1.04
The published continuous-infusion target
| Source | Target | Basis | Strength |
|---|---|---|---|
| SFPT/SFAR 2018, recommendation R2.2 | free plasma concentration 4-8 × MIC for 100% of the dosing interval | Free drug; the causative organism’s MIC | Expert suggestion. The guideline applied GRADE to the literature but assigns R2.2 no graded level, noting too few studies powered for mortality; the supporting evidence is animal, in-vitro and observational, including the DALI cohort |
| Berry and Kuti 2022, reporting the TARGET trial | an upper limit as well as a lower one | Steady-state concentration, agent-specific | Mortality was higher below 32 mg/L and above 96 mg/L, which argues against treating the target as a floor to be exceeded freely |
| BLING III 2024 and its companion meta-analysis | continuous against intermittent infusion | 90-day mortality | Suggestive, not decisive; the figures are in the section below |
What moves the ratio, and by how much
| Change | Effect on the free Css/MIC ratio |
|---|---|
| Double the daily dose | Doubles it |
| Double the clearance (augmented renal clearance) | Halves it |
| Halve the clearance (renal impairment) | Doubles it |
| MIC one doubling dilution higher | Halves it |
| MIC one doubling dilution lower | Doubles it |
| Protein binding falls from 95% to 85% | Triples it |
| Protein binding falls from 30% to 20% | Raises it by about a seventh |
When the target stops being a time and becomes a concentration
A continuous infusion removes the peak and the trough. Once steady state arrives the concentration sits flat at the infusion rate divided by the clearance, so the question “how much of the interval is free drug above the MIC?” has only two possible answers: all of it, if the steady-state concentration exceeds the MIC, or none of it, if it does not. The time-based index collapses, and what replaces it is a concentration multiple — how many times the MIC the plateau sits at. The SFPT/SFAR 2018 recommendation puts that in exactly those terms: a free plasma beta-lactam concentration of four to eight times the MIC, maintained for 100% of the dosing interval.
Only one pharmacokinetic parameter enters the answer, and that is worth dwelling on because it is unusual. At steady state the rate in equals the rate out, and the rate out is clearance times concentration, so the plateau is determined by clearance alone. The volume of distribution decides how long steady state takes to arrive — four to five half-lives, which for a beta-lactam is a few hours and for a drug with a long half-life can be days — and it decides the size of the loading dose needed to get there sooner. It does not move the plateau. The practical corollary is that a continuous infusion started without a loading dose spends its first several half-lives below target, which is precisely the window in which a septic patient is least able to afford it; the loading dose calculator sizes that dose, and it does not change in renal impairment even though the infusion rate must.
The answer this page most often gives is a number below four, and that is informative rather than disappointing. At a piperacillin daily dose of 16 g, a clearance of 14 L/h, 30% binding and an MIC of 16 mg/L, the free steady-state concentration is 33 mg/L and the ratio is 2.1. Reaching four would need about 31 g a day and reaching eight about 61 g — neither a licensed dose. The honest conclusion in that situation is that the published target is not reachable with this agent at this MIC, which is a fact about the arithmetic rather than a prompt to prescribe more. Two factors of two sit either side of it: critical illness can double clearance through augmented renal clearance or halve it through acute kidney injury, and the MIC itself carries a doubling-dilution bracket.
Three things this page does not model. Stability in the bag limits continuous infusion for several beta-lactams and is a pharmacy question. The upper end matters as well as the lower: the TARGET trial, as Berry and Kuti report it, found mortality higher below 32 mg/L and above 96 mg/L, so the target is a window rather than a floor. And the evidence that continuous infusion changes outcomes is suggestive rather than settled — BLING III’s 7,031 patients gave 90-day mortality of 24.9% against 26.8%, an unadjusted odds ratio of 0.91 (0.81 to 1.01, P = 0.08) with a prespecified adjusted 0.89 (0.79 to 0.99, P = 0.04), while its companion meta-analysis of 18 trials gave a relative risk of 0.86 (credible interval 0.72 to 0.98). The intermittent comparison is on the extended versus intermittent infusion calculator, and the dose arithmetic itself on the maintenance dose calculator. This page does not hold a breakpoint table. EUCAST and CLSI publish them, they are revised two or three times a year, and the laboratory that issued your report has already applied its own version. Take the MIC and the interpretive category from the report; this page does the arithmetic that sits on top of them.
Frequently asked questions
What is the target concentration for a continuous beta-lactam infusion?
The SFPT/SFAR 2018 recommendation R2.2 suggests a free plasma beta-lactam concentration of 4 to 8 times the MIC of the causative organism, maintained for 100% of the dosing interval. The guideline presents this as expert suggestion resting on animal, in-vitro and observational data; it assigns no graded level, noting that too few studies were powered for mortality.
Why does the volume of distribution not appear?
Because at steady state the rate in equals the rate out, and the rate out is clearance times concentration. The plateau therefore depends on clearance alone. The volume of distribution decides how fast steady state arrives — four to five half-lives — and the size of the loading dose needed to get there sooner, but not where the plateau sits.
Do I still need a loading dose with a continuous infusion?
The plateau is reached after four to five half-lives whatever the infusion rate, so without a loading dose the concentration is below target for that whole period. A loading dose fills the volume of distribution and is independent of clearance, which is why it does not fall in renal impairment even though the infusion rate must.
Is a higher ratio always better?
No, and the evidence points to a window rather than a floor. The TARGET trial, as Berry and Kuti report it, found mortality higher at steady-state concentrations below 32 mg/L and above 96 mg/L. EXPAT found neither 100% fT>MIC nor 100% fT>4×MIC significantly associated with 30-day survival. Beta-lactam neurotoxicity at high concentrations is a real constraint.
What if the calculated ratio is below 4?
Then the published target is not reachable at that dose, clearance and MIC, which is a fact about the arithmetic. The figures beside the answer show the daily dose it would need, frequently larger than any licensed dose. What follows is a question for the treating team and the laboratory, not one this page can answer.
Does continuous infusion improve survival?
The direction is consistent and the largest trial was not conclusive: BLING III and its companion meta-analysis are summarised in the section above, with the odds ratio, the credible interval and the caveat that the primary unadjusted analysis did not reach significance.
Related calculators
References
- Societe Francaise de Pharmacologie et de Therapeutique and Societe Francaise d’Anesthesie et de Reanimation. Recommandations de Pratiques Professionnelles: optimisation du traitement par beta-lactamines chez le patient de soins critiques. 2018; published in English as Guilhaumou R et al, Crit Care. 2019;23:104. Recommendation R2.2.
- Berry AV, Kuti JL. Pharmacodynamic thresholds for beta-lactam antibiotics: a story of mouse versus man. Front Pharmacol. 2022;13:833189. Open access; source for the murine %fT>MIC thresholds and for the human studies set against them.
- Dulhunty JM, Brett SJ, De Waele JJ, et al; BLING III Study Investigators and the ANZICS Clinical Trials Group. Continuous vs intermittent beta-lactam antibiotic infusions in critically ill patients with sepsis: the BLING III randomized clinical trial. JAMA. 2024;332(8):629-637.
- Abdul-Aziz MH, Hammond NE, Brett SJ, et al. Prolonged vs intermittent infusions of beta-lactam antibiotics in adults with sepsis or septic shock: a systematic review and meta-analysis. JAMA. 2024;332(8):638-649.
- ZOSYN (piperacillin and tazobactam) for injection. US prescribing information, Clinical Pharmacology. Source for about 30% binding of both components and a half-life of 0.7 to 1.2 hours.
- Mouton JW, Muller AE, Canton R, Giske CG, Kahlmeter G, Turnidge J. MIC-based dose adjustment: facts and fables. J Antimicrob Chemother. 2018;73(3):564-568. Source for the log2 variability figures, the ISO 20776-2 criterion and the stated conclusion that individual MIC-based dose adjustment is not justified.
- European Committee on Antimicrobial Susceptibility Testing. Breakpoint tables for interpretation of MICs and zone diameters, version 16.1, 2026, stated valid 24 June to 31 December 2026. https://www.eucast.org. Cited rather than reproduced.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/