Figure 327

Clinical application of the transtubular potassium gradient (TTKG). The TTKG in normal persons varies much but is genarally within the the range of 6 to 12. Hypokalemia from extrarenal causes results in renal potassium conservation and a TTKG less than 2. A higher value suggests renal potassium losses, as through hyperaldostero-nism. The expected TTKG during hyperkalemia is greater than 10. An inappropriately low TTKG in a hyperkalemic patient suggests hypoaldosteronism or a renal tubule defect. Administration of the mineralocorticoid 9 a-fludrocortisone (0.05 mg) should cause TTKG to rise above 7 in cases of hypoaldosteronism. Circumstances are listed in which the TTKG would not increase after mineralocorticoid challenge, because of tubular resistance to aldosterone [21].

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