Medical Formulas

Variables

• HCO3: Serum Bicarbonate (mEq/L)

Clinical Use

Determines if respiratory compensation is adequate in metabolic acidosis. If measured pCO2 differs from expected, a mixed acid-base disorder is present.

Variables

• PAO2: Alveolar partial pressure of oxygen
• FiO2: Fraction of inspired oxygen (0.21 on room air)
• Patm: Atmospheric pressure (760 mmHg at sea level)
• PH2O: Vapor pressure of water (47 mmHg)
• PaCO2: Arterial partial pressure of carbon dioxide
• R: Respiratory quotient (typically 0.8)

Clinical Use

Calculates the partial pressure of oxygen in the alveoli. Used to determine the A-a gradient to diagnose causes of hypoxemia.

Variables

• CO: Cardiac Output
• VO2: Oxygen consumption
• Ca: Oxygen content of arterial blood
• Cv: Oxygen content of mixed venous blood

Clinical Use

Gold standard method for calculating cardiac output based on oxygen consumption and extraction.

Variables

• PAO2: Alveolar oxygen (from Alveolar Gas Equation)
• PaO2: Arterial oxygen (from ABG)

Clinical Use

Helps determine the source of hypoxemia. Normal A-a gradient suggests hypoventilation or low FiO2; elevated suggests V/Q mismatch or shunt.

Variables

• U_Na: Urine Sodium
• P_Cr: Plasma Creatinine
• P_Na: Plasma Sodium
• U_Cr: Urine Creatinine

Clinical Use

Differentiates prerenal disease (FENa < 1%) from acute tubular necrosis (FENa > 2%) in patients with acute kidney injury (AKI).

Variables

• CrCl: Creatinine Clearance (mL/min)
• Age: Years
• Weight: Kilograms
• SCr: Serum Creatinine (mg/dL)

Clinical Use

Estimates glomerular filtration rate (GFR) for the purpose of renal drug dosing adjustments.

Variables

• Measured Ca: Serum Total Calcium (mg/dL)
• Albumin: Serum Albumin (g/dL)

Clinical Use

Adjusts total calcium levels in the setting of hypoalbuminemia, as a large portion of calcium is bound to albumin.

Variables

• Measured Na: Serum Sodium (mEq/L)
• Glucose: Serum Glucose (mg/dL)

Clinical Use

Corrects pseudohyponatremia caused by the osmotic pull of water into the extracellular space during severe hyperglycemia.

Variables

• TBW: Total Body Water (Weight in kg × 0.6 for men, 0.5 for women)
• Measured Na: Serum Sodium (mEq/L)

Clinical Use

Estimates the volume of free water required to correct hypernatremia. Should be replaced slowly to prevent cerebral edema.

Variables

• SBP: Systolic Blood Pressure
• DBP: Diastolic Blood Pressure

Clinical Use

Represents average perfusion pressure. A MAP >= 65 mmHg is typically required to adequately perfuse organs (brain, kidneys) in shock.

Variables

• QT: Uncorrected QT interval (seconds)
• RR: R-R interval (seconds)

Clinical Use

Corrects the QT interval for heart rate. Prolonged QTc (>450ms in men, >460ms in women) increases risk for Torsades de Pointes.

Variables

• PaO2: Arterial oxygen tension (from ABG)
• FiO2: Fraction of inspired oxygen (expressed as a decimal, e.g., 0.40)

Clinical Use

Assesses severity of hypoxic respiratory failure and ARDS (Mild: 200-300, Moderate: 100-200, Severe: <100).

Variables

• Na: Serum Sodium
• Cl: Serum Chloride
• HCO3: Serum Bicarbonate

Clinical Use

Used to differentiate causes of metabolic acidosis. High AG suggests unmeasured anions (MUDPILES). Normal range is typically 8-12 mEq/L.

Variables

• Measured AG: Patient's Anion Gap
• Measured HCO3: Patient's Serum Bicarbonate

Clinical Use

Determines if a mixed acid-base disorder is present in the setting of a high anion gap metabolic acidosis.

Variables

• Weight: Kilograms
• % TBSA: Total Body Surface Area burned (whole number, e.g., 20 for 20%)

Clinical Use

Calculates the total volume of Lactated Ringer's needed in the first 24 hours for severe burn patients. Give half in the first 8 hours, and the rest over the next 16 hours.

Variables

• Weight: Kilograms

Clinical Use

Calculates the hourly maintenance IV fluid rate for pediatric and adult patients who cannot tolerate oral intake.

Variables

• Na: Serum Sodium (mEq/L)
• Glucose: Serum Glucose (mg/dL)
• BUN: Blood Urea Nitrogen (mg/dL)

Clinical Use

Estimates the concentration of solutes in the blood. Normal range is typically 275-295 mOsm/kg. Useful in evaluating hyponatremia.

Variables

• Measured Osmolality: From lab (freezing point depression)
• Calculated Osmolality: From formula

Clinical Use

An elevated gap (> 10) suggests the presence of unmeasured osmotically active toxic alcohols (methanol, ethylene glycol, isopropanol).

Variables

• WBC: Total White Blood Cell Count (cells/mcL)
• % Neutrophils: Polys / Segs
• % Bands: Immature neutrophils

Clinical Use

Evaluates immune system status, particularly in chemotherapy patients. An ANC < 500 represents severe neutropenia.

Variables

• Reticulocyte Count: Percentage
• Patient Hct: Patient's Hematocrit
• Normal Hct: Usually 45% for men, 40% for women

Clinical Use

Corrects the reticulocyte count in anemia to determine if bone marrow response is adequate. RI < 2 suggests inadequate production; RI > 3 suggests hemolysis/loss.

Variables

• Weight: Kilograms
• Target Hb: Usually 15 g/dL
• Actual Hb: Patient's Hemoglobin (g/dL)
• Depot Iron: 500mg for adults

Clinical Use

Calculates the total dose of IV iron needed to replete stores and correct iron deficiency anemia.

Variables

• Weight: Kilograms
• Height: Meters

Clinical Use

Standard method for classifying underweight, healthy weight, overweight, and obesity.

Variables

• Height: Centimeters
• Weight: Kilograms

Clinical Use

Used for determining precise dosing of chemotherapy, glucocorticoids, and indexing cardiac output.

Variables

• Height: Inches > 5 feet

Clinical Use

Used for tidal volume calculation in ARDS (6mL/kg IBW) and dosing hydrophilic medications.

Variables

• MAP: Mean Arterial Pressure (mmHg)
• ICP: Intracranial Pressure (mmHg) - or CVP if CVP > ICP

Clinical Use

Ensures adequate blood flow to the brain in TBI or elevated ICP. Target CPP is usually 60-70 mmHg.

Variables

• CO: Cardiac Output (L/min)
• BSA: Body Surface Area (m²)

Clinical Use

Normalizes cardiac output to the patient's body size. Normal CI is 2.5 - 4.0 L/min/m². A CI < 2.2 suggests cardiogenic shock.

Variables

• MAP: Mean Arterial Pressure (mmHg)
• CVP: Central Venous Pressure (mmHg)
• CO: Cardiac Output (L/min)

Clinical Use

Measures left ventricular afterload. High in hypovolemic/cardiogenic shock, low in distributive (septic/anaphylactic) shock.

Variables

• mPAP: Mean Pulmonary Artery Pressure (mmHg)
• PAOP: Pulmonary Artery Occlusion (Wedge) Pressure (mmHg)
• CO: Cardiac Output (L/min)

Clinical Use

Measures right ventricular afterload. Elevated in pulmonary hypertension, pulmonary embolism, and ARDS.

Variables

• HCO3: Serum Bicarbonate (mEq/L)
• PaCO2: Arterial CO2 (mmHg)

Clinical Use

Describes the derivation of pH as a measure of acidity in biological and chemical systems. Fundamental to understanding acid-base physiology.

Variables

• FiO2: Fraction of Inspired O2 (expressed as percentage, e.g. 100 for 100%)
• MAP: Mean Airway Pressure (cm H2O) on ventilator
• PaO2: Arterial O2 from ABG

Clinical Use

Used primarily in neonates and pediatrics to assess the severity of hypoxic respiratory failure and the need for ECMO (OI > 40).

Variables

• Total Cholesterol: mg/dL
• HDL: High-Density Lipoprotein (mg/dL)
• Triglycerides: mg/dL

Clinical Use

Estimates LDL cholesterol. Not valid if Triglycerides are > 400 mg/dL (in which case a direct LDL must be measured).

Variables

• Serum Iron: mcg/dL
• TIBC: Total Iron Binding Capacity (mcg/dL)

Clinical Use

Evaluates iron status. TSAT < 20% indicates absolute or functional iron deficiency; TSAT > 50% suggests iron overload (hemochromatosis).

Variables

• HbA1c: Glycated Hemoglobin (%)

Clinical Use

Translates the HbA1c percentage into an average daily blood glucose level, which is often easier for patients with diabetes to conceptualize.

Variables

• Serum Albumin: g/dL
• Ascitic Fluid Albumin: g/dL (from paracentesis)

Clinical Use

Determines the cause of ascites. SAAG >= 1.1 indicates portal hypertension (cirrhosis, heart failure). SAAG < 1.1 indicates non-portal causes (malignancy, TB, pancreatitis).

Variables

• ARR: Control Event Rate (CER) - Experimental Event Rate (EER)

Clinical Use

Estimates how many patients need to be treated with a specific intervention to prevent one additional bad outcome.

Variables

• ARI: Experimental Event Rate (EER) - Control Event Rate (CER)

Clinical Use

Estimates how many patients need to be exposed to a risk factor (or treatment) to cause harm in one patient.

Variables

• TP: True Positives
• FN: False Negatives

Clinical Use

The probability that a test will be positive in a patient WITH the disease. Highly sensitive tests (SN-OUT) are used to rule OUT diseases.

Variables

• TN: True Negatives
• FP: False Positives

Clinical Use

The probability that a test will be negative in a patient WITHOUT the disease. Highly specific tests (SP-IN) are used to rule IN diseases.

Variables

• TP: True Positives
• FP: False Positives

Clinical Use

The probability that a patient with a positive test actually has the disease. Highly dependent on the prevalence of the disease in the population.

Variables

• TN: True Negatives
• FN: False Negatives

Clinical Use

The probability that a patient with a negative test actually does not have the disease. Highly dependent on disease prevalence.

Variables

• Cases: Individuals with the disease
• Controls: Individuals without the disease

Clinical Use

Primarily used in Case-Control studies to determine the odds that an outcome will occur given a particular exposure.

Variables

• U_Na: Urine Sodium (mEq/L)
• U_K: Urine Potassium (mEq/L)
• U_Cl: Urine Chloride (mEq/L)

Clinical Use

Used in normal-gap metabolic acidosis to differentiate renal (RTA) vs GI (diarrhea) loss of bicarbonate. A negative UAG suggests GI loss.

Variables

• U_K: Urine Potassium
• P_K: Plasma Potassium
• U_Osm: Urine Osmolality
• P_Osm: Plasma Osmolality

Clinical Use

Estimates the aldosterone-driven potassium secretion in the cortical collecting duct. Useful for evaluating hyperkalemia and hypokalemia.

Variables

• Measured Phenytoin: mcg/mL
• Albumin: Serum Albumin (g/dL)

Clinical Use

Phenytoin is highly protein-bound. In patients with low albumin or renal failure, the measured total level underestimates the active (free) drug level.