Transtubular Potassium Gradient (TTKG) Calculator

Facilitates the differential diagnosis between hyperkalemia and hypokalemia.

Read more about the formula used and about how the results are interpreted in the text below the form.


The transtubular potassium gradient calculator evaluates the conservation of potassium in the collecting ducts of the kidney.

By comparing the K concentration in the kidney ducts with that from the peritubular capillaries, TTKG can help with the differential diagnosis of hyperkalemia and hypokalemia.


The transtubular potassium gradient formula is based on urine and plasma determinations:

TTKG = (Urine K x Plasma osm) / (Plasma K x Urine osm)

The normal TTKG values range between 8 and 9 in healthy patients with a normal diet.


Plasma osmolality (Posm):*
Plasma K (PK):*
Urine osmolality (Uosm):*
Urine K (UK):*
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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


 

TTKG explained

The above calculator analyses the ratio of potassium located in the lumen of the kidney cortical collecting ducts (CCD) compared to that of potassium in the peritubular capillaries.

This offers information on the K conservation and can further inform hyperkalemia and hypokalemia diagnosis.

The variables used to determine the TTKG are defined in the table below:

Variable Measurement unit Description
Urine K mEq/L Clinically assessed as spot urine test. Low urine K (below 40 mEq/L) indicates some form of renal loss.
Plasma osmolality mOsm/kg Urine to plasma ratio is used to account and adjust for water reabsorbing processes from the medullary region of the kidney.
Plasma K mEq/L Urine to plasma K ratio defines the gradient for K secretion.
Urine osmolality mOsm/kg When greater than 700 mOsm/kg means kidneys excrete too much K.

The formula used is:

TTKG = (Urine K x Plasma osm) / (Plasma K x Urine osm)

There are some restrictions applying to the validity of this formula. When urine osmolarity is above 300 mOsm/kg and urine sodium is above 25 mEq/L the formula may not be reliable enough for use because it doesn’t exclude the occurrence of a possible potassium wasting syndrome.

The TTKG is based on a study conducted by Ethier et al. in 1990. The study was aimed at defining expected values for a parameter (TTKG) in patients with hypokalemia or following an acute K load, and led to the creation of a semi-quantitative index of the activity of the K secretory process.

It was found that the expected value for the TTKG must be interpreted relative to the concentration of K in the plasma. Also, during a water diuresis and pre-existing hypokalemia, TTKG values tend to be low, despite hyperaldosteronism.

 

Result interpretation

The general guideline is that TTKG values between 8 and 9 are considered normal in patients with a normal diet.

However, when there is high potassium intake in the diet, it may rise even above 10 without indication of hyperkalemia.

Similarly, with low potassium intake, TTKG may appear to be lower than normal, without indication of hypokalemia.

The transtubular potassium gradient can also offer information as to how well the kidneys are adjusting to the potassium processes in the collecting tube.

Where high TTKG values are accompanied by hypokalemia, diagnosis of hyperaldosteroinism or Liddle’s syndrome may be likely.

Values below 7 but with high potassium levels, are indicative of hyperkalemia and when accompanying sodium and high urine Na, mineralocorticoid deficiency is likely to be diagnosed.

Potassium depletion or hypokalemia is to be considered for TTKGs of below 3 where there is a reduced urinary excretion of K.

Future research is to address the positive correlation between the mineralocorticoid activity and the tubular gradient index and how this relation can be used in renal diagnosis.

 

Original source

Ethier JH, Kamel KS, Magner PO, Lemann J Jr, Halperin ML. The transtubular potassium concentration in patients with hypokalemia and hyperkalemia. Am J Kidney Dis 1990; 15(4):309-15.

Other references

1. Joo KW, Chang SH, Lee JG, Na KY, Kim YS, Ahn C, Han JS, Kim S, Lee JS. Transtubular potassium concentration gradient (TTKG) and urine ammonium in differential diagnosis of hypokalemia. J Nephrol. 2000; 13(2):120-5.

2. Choi MJ, Ziyadeh FN. The utility of the transtubular potassium gradient in the evaluation of hyperkalemia. J Am Soc Nephrol. 2008; 19(3):424-6.


App Version: 1.0.1

Coded By: MDApp

Specialty: Nephrology

System: Urinary

Objective: Determination

Type: Calculator

No. Of Variables: 4

Year Of Study: 1990

Abbreviation: TTKG

Article By: Denise Nedea

Published On: May 23, 2017 · 02:08 PM

Last Checked: May 23, 2017

Next Review: May 23, 2018