Clinical Calculators

High-yield formulas and scores — live-computed, nothing stored. Tested on all three Steps and used every day on rotations.

Anion Gap

Acid–base

Serum anion gap to work up a metabolic acidosis.

Sodium (Na⁺)

mmol/L

Chloride (Cl⁻)

mmol/L

Bicarbonate (HCO₃⁻)

mmol/L

AG = Na − (Cl + HCO₃)

12 mmol/L

Normal (8–12).

Winter's Formula

Acid–base

Expected PaCO₂ for respiratory compensation of a metabolic acidosis.

Bicarbonate (HCO₃⁻)

mmol/L

Expected PaCO₂ = 1.5 × HCO₃ + 8 (±2)

24–28 mmHg

Measured PaCO₂ above range → concurrent respiratory acidosis; below → respiratory alkalosis.

Corrected Calcium

General

Adjust total calcium for hypoalbuminemia.

Measured calcium

mg/dL

Albumin

g/dL

Corrected Ca = Ca + 0.8 × (4 − albumin)

9.6 mg/dL

Normal (8.5–10.5).

Corrected Sodium

General

Adjust measured sodium for hyperglycemia.

Measured sodium

mmol/L

Glucose

mg/dL

Corrected Na = Na + 0.016 × (glucose − 100)

138 mmol/L

True Na after correcting the dilutional effect of glucose.

A–a Gradient

Pulm

Alveolar–arterial oxygen gradient to classify hypoxemia.

PaO₂ (arterial)

mmHg

PaCO₂

mmHg

FiO₂ (0.21 = room air)

Age

A–a = [FiO₂ × (760 − 47) − PaCO₂/0.8] − PaO₂

29.7 mmHg

Expected ≈ 14 (age/4 + 4). Elevated → V/Q mismatch, shunt, or diffusion defect.

Creatinine Clearance (Cockcroft–Gault)

Renal

Estimate CrCl for renal drug dosing.

Age

Weight

Serum creatinine

mg/dL

Sex

CrCl = [(140 − age) × wt × (0.85 if ♀)] / (72 × SCr)

77.8 mL/min

Adequate clearance.

Fractional Excretion of Sodium (FENa)

Renal

Distinguish pre-renal AKI from ATN.

Urine sodium

mmol/L

Plasma sodium

mmol/L

Urine creatinine

mg/dL

Plasma creatinine

mg/dL

FENa = (U_Na × P_Cr) / (P_Na × U_Cr) × 100

0.14 %

< 1% → pre-renal (or contrast/GN).

CHA₂DS₂-VASc

Cardio

Stroke risk in non-valvular atrial fibrillation.

CHF / LV dysfunction

Hypertension

Age ≥ 75

Diabetes

Prior stroke/TIA/thromboembolism

Vascular disease (MI/PAD/aortic plaque)

Age 65–74

Female sex

0 points

Low risk — no anticoagulation.

Wells Score (PE)

Pulm

Pre-test probability of pulmonary embolism.

Clinical signs of DVT

PE is #1 diagnosis (or equally likely)

Heart rate > 100

Immobilization ≥3 d / surgery in 4 wk

Prior DVT/PE

Hemoptysis

Malignancy (treated within 6 mo)

0 points

Low → D-dimer (PERC if applicable).

CURB-65

Pulm

Severity + disposition for community-acquired pneumonia.

Confusion (new)

BUN > 19 mg/dL (urea > 7 mmol/L)

Respiratory rate ≥ 30

SBP < 90 or DBP ≤ 60

Age ≥ 65

0 points

Low — outpatient likely.

MELD-Na

Hepatic

90-day mortality in end-stage liver disease / transplant priority.

Bilirubin

mg/dL

INR

Creatinine

mg/dL

Sodium

mmol/L

MELD(i) with Na correction

MELD 15 → MELD-Na 17. Higher = greater short-term mortality / transplant priority.

Glasgow Coma Scale

Neuro/Trauma

Level of consciousness after head injury.

Eye opening

Verbal response

Motor response

15 / 15

13–15 → mild.

Maintenance IV Fluids (4-2-1)

OB/Peds

Hourly maintenance fluid rate by weight.

Weight

4 mL/kg (first 10) + 2 (next 10) + 1 (rest)

70 mL/hr

≈ 1680 mL/day. (Holliday–Segar 4-2-1 rule.)

BMI + Body Surface Area

General

Body mass index and Mosteller BSA.

Weight

Height

BMI = kg/m²; BSA = √(ht·wt / 3600)

BMI 22.9 · BSA 1.84 m²

normal (BMI).

APGAR Score

OB/Peds

Newborn status at 1 and 5 minutes.

Appearance (color)

Pulse

Grimace (reflex)

Activity (tone)

Respiration

10 / 10

Reassuring.

Ranson's Criteria (admission)

Severity of acute (non-gallstone) pancreatitis at admission.

Age > 55

WBC > 16,000 /mm³

Glucose > 200 mg/dL

AST > 250 U/L

LDH > 350 U/L

GA-LAW: Glucose, Age, LDH, AST, WBC

0 / 5 at admission

Lower admission risk — reassess the 48-hour criteria.

Corrected Reticulocyte / RPI

Heme

Is the marrow responding appropriately to anemia?

Reticulocyte %

Hematocrit

Corrected = retic% × (Hct/45); RPI = corrected ÷ maturation factor

Corrected 1.3% · RPI 0.9

RPI <2 → hypoproliferative (marrow/iron/B12/EPO).

Parkland Formula (burns)

General

24-hour crystalloid resuscitation for major burns.

Weight

% TBSA burned

4 mL × %TBSA × weight(kg); half over the first 8 h

8400 mL / 24 h

4200 mL over first 8 h (525 mL/hr), rest over next 16 h. Lactated Ringer's; titrate to urine output.

Corrected QT (Bazett)

Cardio

Rate-corrected QT — torsades risk.

QT interval

Heart rate

bpm

QTc = QT ÷ √RR (RR = 60 / HR, in seconds)

400 ms

Normal.

Free Water Deficit

Renal

Water deficit to correct hypernatremia.

Weight

Serum sodium

mmol/L

TBW fraction

TBW × (Na/140 − 1); TBW = weight × factor

6 L

Correct slowly — lower Na ≤10–12 mmol/L per 24 h to avoid cerebral edema.

Educational reference only. Ranges/cutoffs are common teaching values; confirm against your institution and clinical judgment.