When is the TTKG not reliable?

The TTKG requires concentrated urine (U_Osm > S_Osm) and adequate sodium delivery to the distal nephron (U_Na > 25 mEq/L). In the setting of water diuresis (dilute urine) or very low sodium intake, the TTKG is unreliable. Some nephrologists argue the TTKG has fundamental limitations and prefer 24-hour urine K⁺ or urine K/creatinine ratio.

What are the most common causes of low TTKG with hyperkalemia?

Type 4 renal tubular acidosis (hypoaldosteronism, very common in diabetic nephropathy), ACE inhibitors, ARBs, K⁺-sparing diuretics (spironolactone, amiloride, triamterene), TMP-SMX, heparin, calcineurin inhibitors, and adrenal insufficiency.

Can I use TTKG to diagnose primary hyperaldosteronism?

Indirectly. In hypokalemia with a TTKG > 3, the kidney is wasting potassium, which can be from excess aldosterone. However, the aldosterone-to-renin ratio (ARR) is the specific screening test. TTKG distinguishes renal from GI causes but does not specify the renal mechanism.

What if TTKG is borderline (5-7 in hyperkalemia)?

Borderline values are common in chronic kidney disease where nephron mass reduction impairs K⁺ excretion even with adequate aldosterone. Consider checking an aldosterone level and evaluating GFR. A fludrocortisone trial may help differentiate aldosterone deficiency from tubular resistance.

Why is the TTKG controversial?

Some researchers (including the original authors) have questioned whether the TTKG accurately reflects collecting duct K⁺ secretion because urea recycling in the medulla can alter the osmolality correction. Despite this, TTKG remains widely used in clinical practice because no simple alternative exists for bedside assessment of renal K⁺ handling.

What is an alternative to TTKG?

A 24-hour urine potassium collection is more robust: > 30 mEq/day in hypokalemia indicates renal wasting. A spot urine K⁺/creatinine ratio > 15 mEq/g also suggests renal loss. These avoid the osmolality assumptions inherent in the TTKG calculation.

TTKG Calculator — Transtubular Potassium Gradient

Calculate the transtubular potassium gradient (TTKG) as a renal-potassium-handling worksheet for hyperkalemia or hypokalemia review, with validity checks and limitation notes.

⚠️ Clinical Note: The TTKG assumes urine osmolality is greater than serum osmolality and that urine sodium is > 25 mEq/L (adequate sodium delivery). Results are unreliable if these conditions are not met. Some nephrologists have raised concerns about TTKG validity — always interpret in full clinical context.

Clinical Scenario

Clinical Context

Laboratory Values

Serum Potassium (mEq/L)

Urine Potassium (mEq/L)

Serum Osmolality (mOsm/kg)

Urine Osmolality (mOsm/kg)

Serum Sodium (mEq/L)

TTKG

3.2

Inappropriately low — impaired renal K⁺ excretion (hypoaldosteronism, tubular defect, or K⁺-sparing diuretic)

Adjusted Urine K⁺

19.3 mEq/L

Corrected for water reabsorption

Serum K⁺

6 mEq/L

Hyperkalemia

Validity Check

✓ Valid

Urine Osm > Serum Osm met

TTKG = 3.2

Inappropriately low — impaired renal K⁺ excretion (hypoaldosteronism, tubular defect, or K⁺-sparing diuretic)

TTKG Interpretation Reference

Scenario	TTKG	Interpretation	Common Causes
Hyperkalemia (K⁺ > 5.0 mEq/L)
HyperK	< 5	Impaired K⁺ excretion	Hypoaldosteronism (type 4 RTA), K⁺-sparing diuretics, ACEi/ARB, TMP-SMX, heparin
HyperK	5–7	Borderline	Partial aldosterone deficiency, CKD with reduced nephron mass
HyperK	> 7	Appropriate response	K⁺ load (diet, supplements, blood products), rhabdomyolysis, tumor lysis, acidosis shift
Hypokalemia (K⁺ < 3.5 mEq/L)
HypoK	< 2	Appropriate conservation	GI loss (diarrhea, vomiting), poor intake, transcellular shift (insulin, β-agonists)
HypoK	2–3	Borderline renal loss	Early diuretic effect, mild hyperaldosteronism
HypoK	> 3	Renal K⁺ wasting	Loop/thiazide diuretics, hyperaldosteronism, Bartter/Gitelman, RTA type 1&2, Mg depletion

Differential Diagnosis Guide

Condition	K⁺	TTKG	Key Features
Primary hyperaldosteronism	Low	> 3	Hypertension, metabolic alkalosis, low renin
Type 4 RTA (hypoaldo)	High	< 5	Mild CKD, diabetes, ACEi/ARB use, mild acidosis
Loop diuretic use	Low	> 3	Metabolic alkalosis, volume contraction
GI losses (diarrhea)	Low	< 2	Volume depletion, metabolic acidosis or alkalosis
Acute K⁺ load	High	> 7	Rhabdomyolysis, TLS, massive transfusion

Planning notes, formulas, and examples

About the TTKG Calculator — Transtubular Potassium Gradient

The transtubular potassium gradient (TTKG) is a traditional bedside estimate of distal potassium handling. It uses serum and urine potassium together with osmolality to ask whether the kidney is appearing to conserve potassium or secrete it in the expected direction for the clinical scenario.

The index is most useful as a structured review aid rather than as a stand-alone answer. It has practical limitations, including its dependence on concentrated urine and adequate distal sodium delivery, and many nephrology workflows now pair it with spot urine potassium indices or 24-hour urine data when available.

This calculator computes the TTKG, checks the usual validity prerequisites, and keeps the hyperkalemia and hypokalemia interpretation bands visible as a worksheet for broader electrolyte review.

When This Page Helps

TTKG can be helpful when you want a quick, structured look at whether the kidney appears to be responding to a potassium disorder in the expected direction. The calculation is fast, but it works best as part of the larger electrolyte picture rather than as a single decisive test.

How to Use the Inputs

Select the clinical scenario (hyperkalemia or hypokalemia workup).
Enter serum potassium and urine potassium in mEq/L.
Enter serum osmolality and urine osmolality in mOsm/kg.
Enter serum sodium for osmolality cross-reference.
Review the TTKG value, validity check, and clinical interpretation.
Consult the differential diagnosis tables for cause identification.

Formula used

TTKG = (U_K / S_K) / (U_Osm / S_Osm) = (U_K × S_Osm) / (S_K × U_Osm)
Validity: requires U_Osm > S_Osm and U_Na > 25 mEq/L
Hyperkalemia: TTKG < 5 = impaired excretion; > 7 = appropriate
Hypokalemia: TTKG < 2 = appropriate conservation; > 3 = renal wasting

Example Calculation

Result: TTKG = 2.3 — Impaired renal K⁺ excretion

TTKG = (25 × 290) / (6.2 × 500) = 7250 / 3100 = 2.3. In the setting of hyperkalemia (K⁺ 6.2), a TTKG of 2.3 is inappropriately low (expected > 7). This indicates the kidneys are not adequately secreting potassium, suggesting hypoaldosteronism, K⁺-sparing diuretic effect, or tubular defect.

Tips & Best Practices

Always check the validity prerequisites: urine osmolality must exceed serum osmolality and urine sodium should be > 25 mEq/L.
Interpret TTKG only after confirming that the urine is concentrated enough for the index to be meaningful.
When the prerequisites are not met, a spot urine potassium-creatinine ratio or a timed urine collection is often more useful.
Remember that concurrent magnesium depletion impairs renal potassium conservation — always check and correct magnesium in hypokalemia.
When TTKG is unreliable (dilute urine), fall back on 24-hour urine potassium or urine K⁺/creatinine ratio.

What the Index Is Trying to Show

TTKG uses the urine-to-serum potassium relationship, adjusted by osmolality, to approximate whether distal potassium secretion is low, appropriate, or excessive for the scenario. In practice it is mainly a quick organizing tool for potassium-disorder review.

Why the Validity Checks Matter

The calculation depends on concentrated urine and adequate distal sodium delivery. If the urine is dilute or distal sodium delivery is very low, the TTKG can look reassuring or abnormal for the wrong reason. That is why the page surfaces the prerequisite checks before leaning on the interpretation bands.

How to Use a Borderline Result

Borderline values should usually push the review back toward the full potassium workup: medication list, acid-base status, kidney function, blood pressure pattern, renin-aldosterone data when relevant, and direct urine potassium measurements. The TTKG is most useful when it supports that broader pattern rather than when it is asked to settle the question by itself.

Sources & Methodology

Last updated: March 28, 2026

Methodology

This calculator computes TTKG as (urine potassium × serum osmolality) / (serum potassium × urine osmolality). It also surfaces the common validity checks by reminding the user that concentrated urine and adequate distal sodium delivery are needed before the number is interpreted with confidence.

The page keeps the hyperkalemia and hypokalemia cutoffs visible as worksheet context, but it does not treat the index as a stand-alone diagnostic or treatment rule.

Sources

The utility of the transtubular potassium gradient in the evaluation of hyperkalemia (PubMed / Clinical Journal of the American Society of Nephrology) — Review discussing the uses and limitations of TTKG in potassium disorders.

Frequently Asked Questions

The TTKG requires concentrated urine (U_Osm > S_Osm) and adequate sodium delivery to the distal nephron (U_Na > 25 mEq/L). In the setting of water diuresis (dilute urine) or very low sodium intake, the TTKG is unreliable. Some nephrologists argue the TTKG has fundamental limitations and prefer 24-hour urine K⁺ or urine K/creatinine ratio.