Q: When is corrected calcium clinically important?

Albumin-corrected calcium adjusts for low albumin: corrected Ca = measured Ca + 0.8 × (4.0 − albumin). This matters because roughly 45% of serum calcium is albumin-bound; hypoalbuminemia falsely lowers total calcium. However, ionized calcium remains normal in pure hypoalbuminemia. If calcium abnormality is clinically suspected, always check ionized calcium rather than relying solely on the correction formula.

Q: What is the osmolar gap and when is it elevated?

The osmolar gap = measured osmolality − calculated osmolality (2×Na + glucose/18 + BUN/2.8). Normal gap is <10 mOsm/kg. An elevated osmolar gap (>10–15) suggests unmeasured osmoles, classically from toxic alcohols: ethanol, methanol, ethylene glycol, isopropanol, or propylene glycol. Always interpret with clinical context — ethanol is the most common cause and can be corrected for in the calculation.

Question 1

What eGFR equation is recommended by KDIGO?

Accepted Answer

KDIGO 2024 recommends the CKD-EPI 2021 creatinine equation (race-free) as the primary method for estimating GFR. When more accuracy is needed, particularly for living kidney donor evaluation or when creatinine is unreliable, CKD-EPI cystatin C or the combined creatinine-cystatin C equation is recommended. MDRD is acceptable but less accurate. Cockcroft-Gault is preferred for drug dosing calculations.

Question 2

How do I calculate FENa and what does it mean?

Accepted Answer

FENa (Fractional Excretion of Sodium) = (urine Na × serum Cr) / (serum Na × urine Cr) × 100. FENa <1% suggests prerenal AKI (tubular reabsorption intact). FENa >2% suggests intrinsic renal injury (ATN). Important caveat: FENa can be <1% in contrast nephropathy, myoglobinuria, and early obstruction. Use FEUrea (<35% = prerenal) in patients on diuretics, as loop diuretics increase FENa regardless of volume status.

Question 3

When is corrected calcium clinically important?

Accepted Answer

Albumin-corrected calcium adjusts for low albumin: corrected Ca = measured Ca + 0.8 × (4.0 − albumin). This matters because roughly 45% of serum calcium is albumin-bound; hypoalbuminemia falsely lowers total calcium. However, ionized calcium remains normal in pure hypoalbuminemia. If calcium abnormality is clinically suspected, always check ionized calcium rather than relying solely on the correction formula.

Question 4

What is the osmolar gap and when is it elevated?

Accepted Answer

The osmolar gap = measured osmolality − calculated osmolality (2×Na + glucose/18 + BUN/2.8). Normal gap is <10 mOsm/kg. An elevated osmolar gap (>10–15) suggests unmeasured osmoles, classically from toxic alcohols: ethanol, methanol, ethylene glycol, isopropanol, or propylene glycol. Always interpret with clinical context — ethanol is the most common cause and can be corrected for in the calculation.

Question 5

How does the transtubular potassium gradient (TTKG) help in hyperkalemia workup?

Accepted Answer

TTKG estimates aldosterone activity in the collecting duct. TTKG = (urine K / serum K) / (urine osmolality / plasma osmolality). In hyperkalemia, a TTKG <5 suggests hypoaldosteronism or aldosterone resistance (type IV RTA, adrenal insufficiency). TTKG >5–7 in hyperkalemia suggests appropriate aldosterone response with excess potassium load or shift. Reliability requires urine osmolality ≥300 and urine Na >25 mEq/L.

Medical Calculators for Nephrology and Renal Medicine

Curated Tools for Nephrology & Renal Medicine (12)

eGFR Calculator

Creatinine Clearance

FENa Calculator

Anion Gap

Osmolar Gap

Corrected Calcium

Free Water Deficit

TTKG

Urine Anion Gap

BUN/Cr Ratio

Corrected Sodium

FEUrea Calculator

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