Healthcare & Clinical
Transtubular Potassium Gradient (TTKG) & Urine Anion Gap (UAG)
Calculate aldosterone-mediated Transtubular Potassium Gradient TTKG = (U_K / P_K) / (U_Osm / P_Osm) (valid when U_Osm > P_Osm) and Urine Anion Gap UAG = U_Na + U_K - U_Cl for renal tubular acidosis.
Assess cortical collecting duct K+ secretion and urinary NH4+ excretion!
Raw spot urine potassium U_K can be misleading when antidiuretic hormone (ADH) concentrates medullary water! Dividing the urine-to-plasma potassium ratio (U_K / P_K) by the osmolality concentration ratio (U_Osm / P_Osm) yields the Transtubular Potassium Gradient TTKG = (U_K * P_Osm) / (P_K * U_Osm)—valid strictly when urine is hyperosmolar to plasma (U_Osm > P_Osm) and distal Na+ delivery U_Na > 25 mEq/L!
Alongside TTKG, nephrologists evaluate normal-anion-gap hyperchloremic metabolic acidosis using the Urine Anion Gap: UAG = U_Na + U_K - U_Cl! In GI bicarbonate loss (diarrhea), healthy kidneys excrete abundant NH4Cl, making U_Cl exceed U_Na + U_K (negative UAG = -20 to -50 mEq/L); in Distal Type 1 Renal Tubular Acidosis (RTA), impaired NH4+ excretion turns UAG positive!
| Electrolyte Workup Metric | Lab Inputs | Calculated Value | Physiological Interpretation |
|---|---|---|---|
| TTKG in Hyperkalemia (Intact Aldosterone) | UK=45, PK=5.5, UOsm=600, POsm=300 | 4.09 | Collecting duct secreting K+ (check intake/shift) |
| TTKG in Hypoaldosteronism / Type 4 RTA | UK=12, PK=5.5, UOsm=600, POsm=300 | 1.09 (< 3!) | Blunted renal K+ excretion |
| Urine Anion Gap (Diarrhea vs Distal RTA) | UNa=40, UK=45, UCl=105 | -20 mEq/L | Negative UAG -> Intact GI-loss NH4+ response |
Transtubular Potassium Gradient TTKG (with Osmolality Validity Guard)
Enforces U_Osm > P_Osm (B4 > B5) because water reabsorption assumption fails in hypotonic urine, then evaluates (U_K / P_K) / (U_Osm / P_Osm).
=IF(OR(B3 <= 0, B5 <= 0), "Invalid Plasma Input", IF(B4 <= B5, "Invalid TTKG (U_Osm Must Exceed P_Osm)", ROUND((B2 / B3) / (B4 / B5), 2)))Urine Anion Gap UAG = U_Na + U_K - U_Cl (mEq/L)
Computes U_Na (B6) + U_K (B2) - U_Cl (B7) to distinguish extrarenal diarrhea (negative UAG) from distal renal tubular acidosis (positive UAG).
=B6 + B2 - B7Try the example
Use the original values or change them to test your understanding.