Clinical Calculators

Creatinine clearance Cockcroft-Gault

Still the equation most drug dosing references are built on, which is why it refuses to go away even though eGFR is a better measure of kidney function. When a package insert tells you to adjust for a creatinine clearance under 30, it means this number.

FENa and FEUrea Prerenal or intrinsic

The fractional excretion of sodium separates a kidney that is holding on to sodium because it is underperfused from one that has lost the ability to hold on to anything.

The catch that matters: a patient on a loop diuretic will have a high FENa no matter how prerenal they are, because you have chemically forced sodium into the urine. In anyone on furosemide, use the fractional excretion of urea instead, substituting urea for sodium in the same equation. A FEUrea under 35 percent suggests prerenal. Diuretics do not affect it.

Also remember FENa is low in several intrinsic causes: contrast nephropathy, rhabdomyolysis, early glomerulonephritis and hepatorenal syndrome all give a low FENa. It is a clue, not a diagnosis.

Corrected sodium for hyperglycemia

Glucose is osmotically active and drags water into the plasma, diluting the measured sodium. In DKA and hyperosmolar states the reported sodium always understates the true sodium, and correcting it changes how you think about the free water deficit and about how the sodium will behave once you start insulin.

The practical consequence: as you treat the hyperglycemia, the measured sodium will rise even though you have not given any sodium. That is expected and it is not a complication. What matters is whether the corrected sodium is falling appropriately, and the corrected number is the one to follow. See the free water deficit calculator for the rest.

Corrected QT interval

The QT interval shortens as the heart rate rises, so a raw QT tells you nothing without correcting for rate. This is the number that decides whether you can safely give another QT-prolonging drug, and on an inpatient service that question comes up constantly.

Bazett is the one printed on most machine reads and the one most protocols are written around, but it overcorrects at fast rates and undercorrects at slow ones. In a tachycardic patient a Bazett QTc will look alarmingly long when Fridericia does not. Both are shown above. When they disagree substantially at a heart rate over 100, Fridericia is closer to the truth.

Before you blame a drug, correct the potassium and the magnesium. Hypokalemia and hypomagnesemia prolong the QT and are far easier to fix than a medication list.

Mean arterial pressure

MAP, not systolic pressure, is what perfuses organs. It is the number resuscitation targets are written around, usually 65 mm Hg as a floor.

Diastole is weighted twice because at normal heart rates the heart spends roughly twice as long in diastole as in systole. At high heart rates that ratio changes and the estimate drifts, which is one reason an arterial line reports a measured MAP rather than a calculated one.