Transtubular Potassium Gradient (TTKG) Calculator

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:

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.