This adjusted phenytoin calculator corrects a measured total phenytoin level for low albumin using the Sheiner-Tozer equation — because phenytoin rides on albumin, and when albumin falls, the same total level hides a much higher free (active) level. It reports both the standard correction and the end-stage renal disease variant, against the usual total therapeutic range of 10–20 mcg/mL.

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Why the Lab Number Lies When Albumin Falls

Phenytoin circulates about 90% bound to albumin; the free 10% crosses into the brain, does the therapeutic work, and causes the toxicity. Routine assays measure total (bound + free). In a patient with normal albumin (~4.0 g/dL) the total is an honest proxy — 10–20 mcg/mL total corresponds to 1–2 free. But albumin craters in critical illness, nephrotic syndrome, liver disease, malnutrition, pregnancy and old age; binding sites vanish, the free fraction can double or triple, and a “therapeutic” total of 12 can be functioning like 24 — a toxic level wearing a reassuring costume. Sheiner and Tozer's 1978 equation rescales the measured total into the equivalent level at normal binding: adjusted = measured ÷ ((0.2 × albumin) + 0.1). In end-stage renal disease, uremic toxins further displace phenytoin from albumin, and the widely used ESRD variant replaces 0.2 with 0.25 — the two settings this calculator offers.

The worked defaults tell the canonical story: measured 12 mcg/mL (calmly mid-range) at albumin 2.0 g/dL adjusts to 24.0 mcg/mL by the standard equation, 20.0 mcg/mL by the ESRD variant — at or above the toxicity threshold either way, in a patient whose chart says “level fine.” This is the clinical scenario the equation exists for, and why guidelines trigger the adjustment when albumin drops below roughly 3.0–3.5 g/dL (or whenever the clinical picture and the total disagree). Where a free (unbound) phenytoin level is measurable, it supersedes all estimates — the equation is a model, the free level is a measurement — and modern practice increasingly orders free levels directly in ICU and renal populations precisely because the adjustment's accuracy is only fair.

Worked Examples (Verified)

  • Defaults: 12 ÷ ((0.2 × 2.0) + 0.1) = 12 ÷ 0.5 = 24.0 mcg/mL (standard); ESRD: 12 ÷ ((0.25 × 2.0) + 0.1) = 12 ÷ 0.6 = 20.0 mcg/mL. Same patient, same blood draw, two coefficients — knowing which population your patient belongs to is the clinical step the calculator can't do for you.
  • Normal albumin sanity check: at albumin 4.0 the standard denominator is (0.8 + 0.1) = 0.9, so a measured 15 adjusts to 16.7 — near-identity, as it should be when binding is near normal; the equation's action concentrates where albumin is low.
  • The deep hypoalbuminemia case: albumin 1.5, measured 8 (“subtherapeutic — increase dose?”): adjusted = 8 ÷ 0.4 = 20.0 — already at the range's ceiling; the correct move is a free level and clinical review, not a dose increase into toxicity.

Phenytoin Discipline Around the Number

  • Timing first: levels are interpreted as troughs (just before a dose) at steady state; a level drawn two hours post-dose or three days after a dose change is a different (often meaningless) number that no albumin formula can rescue. Phenytoin's half-life stretches as levels rise (saturable Michaelis-Menten kinetics) — steady state can take 2–3 weeks.
  • Nonlinear means small steps: near saturation, a 100 mg increase can double the level; adjustments of 30–50 mg with re-checks are the standard of care the textbooks plead for.
  • Displacers and inhibitors stack: valproate both displaces phenytoin from albumin and inhibits its metabolism — total levels can fall while free levels rise; salicylates and sulfonamides displace too. The equation models albumin only, not the drug-interaction layer.
  • Signs beat numbers: nystagmus → ataxia → diplopia/slurred speech → encephalopathy is the classic toxicity ladder; a symptomatic patient with an “in-range” adjusted level is a toxic patient whose estimate failed — treat the patient, verify with a free level.
  • Formulation traps: phenytoin sodium vs acid differ ~8% in content; IV fosphenytoin is dosed in phenytoin equivalents (PE) — level interpretation must use the right drug and route before the albumin math even starts.

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Frequently Asked Questions

How do you calculate adjusted phenytoin?

With the Sheiner-Tozer equation: adjusted = measured total ÷ ((0.2 × albumin) + 0.1), albumin in g/dL. At a measured 12 mcg/mL and albumin 2.0: 12 ÷ 0.5 = 24.0 mcg/mL. For end-stage renal disease, the commonly used modified coefficient is 0.25: 12 ÷ 0.6 = 20.0 mcg/mL. Compare the adjusted value against the standard total range (10–20 mcg/mL); this calculator computes both variants and interprets the result for you.

Why does albumin affect phenytoin levels?

Because ~90% of circulating phenytoin is bound to albumin and only the free fraction is active. Total assays count bound + free together, assuming normal binding. When albumin falls, binding sites fall, free fraction rises — so a given total represents more active drug. The patient with albumin 2.0 and a total of 12 has the free exposure of a normal-albumin patient at ~24: the equation simply converts the reading into the normal-binding equivalent.

When should the adjusted level be used?

Whenever albumin is low (commonly below ~3.0–3.5 g/dL) or binding is otherwise disturbed: critical illness, nephrotic syndrome, cirrhosis, malnutrition, pregnancy, advanced age, burns, and renal failure — plus whenever signs of toxicity or seizures contradict the total level. In those settings, best practice increasingly measures the free (unbound) level directly; the Sheiner-Tozer estimate is the fallback when free levels aren't available, and a good screening step for deciding whether one is needed.

What is the therapeutic range for phenytoin?

Total: 10–20 mcg/mL; free (unbound): 1–2 mcg/mL. Toxicity typically emerges above 20 total-equivalent (nystagmus first), with ataxia and diplopia in the 20–30 zone and encephalopathy beyond ~30–40 — though individuals vary and chronic users tolerate higher levels than acute ones. The “range” is a population guide; seizure control without toxicity at a level of 8, or toxicity at 18 in a low-albumin patient, are both real and are why the number is always read with the patient attached.

Standard or ESRD equation — which applies to my patient?

The standard (0.2) equation for most patients with hypoalbuminemia; the modified (0.25) coefficient for end-stage renal disease, where uremic compounds additionally impair binding. Some institutions use slightly different ESRD coefficients (values up to 0.29 appear in the literature) — one more reason the adjusted number is an estimate with a confidence band, and persistently discordant cases get a measured free level. This calculator labels which variant it applied; protocols vary locally.

How accurate is the Sheiner-Tozer equation?

Fair, not perfect: studies comparing adjusted estimates against measured free levels show the equation improves on the raw total in hypoalbuminemia but still misclassifies a meaningful minority — performing worst in the critically ill and in ESRD, ironically where it's needed most. It's best used as a screen: an adjusted level deep in range is reassuring; an adjusted level near a decision boundary, or discordant with the patient's signs, mandates the free level rather than arithmetic heroics.

What else falsely changes phenytoin interpretation?

Timing (non-trough, pre-steady-state draws), interacting drugs (valproate's dual displacement + inhibition; enzyme inducers like carbamazepine lowering levels), formulation switches (brand/generic and sodium/acid salt differences), and assay interference (fosphenytoin cross-reacts in some immunoassays shortly after IV dosing — levels drawn too soon after fosphenytoin read spuriously high). Interpretation is a checklist, and albumin correction is one line of it; this page's discipline section walks the rest.

Is this adjusted phenytoin calculator free?

Yes — free and instant, reproducing the published Sheiner-Tozer equations as an educational aid; dosing decisions belong to the treating team with levels, timing and patient in hand.