# Tubular Disorders: Localize the Transport Defect

Use blood pressure, potassium, acid–base status, and urinary losses to localize a transport defect.

![Infographic: Tubular Disorders: Localize the Transport Defect](https://urinenephrology.org/visual-reference/images/student-tubulopathies.png?v=20261003c)

## Start with the phenotype

Measure BP, potassium, magnesium, bicarbonate, creatinine, and urine findings. Ask about diuretics, vomiting, supplements, growth, and family history. Acquired drug effects often mimic inherited transport disorders.

## Low/normal BP + hypokalemic alkalosis

Persistent renal chloride loss suggests active diuretics or a salt-wasting tubulopathy. Bartter physiology resembles loop blockade; Gitelman resembles thiazide blockade. Recent diuretic exposure must be excluded before diagnosing an inherited disorder.

## Use magnesium and urine calcium

Gitelman commonly combines hypomagnesemia with low urine calcium. Bartter more often has increased urine calcium or nephrocalcinosis. These clues narrow the differential; they do not establish a genotype.

## High BP + hypokalemic alkalosis

Consider mineralocorticoid or ENaC-mediated sodium retention. Renin/aldosterone patterns help: Liddle physiology suppresses both and calls for ENaC-directed therapy, rather than assuming every case is primary aldosteronism.

## Fanconi: several proximal losses

Normoglycemic glycosuria, phosphate wasting, bicarbonate loss, and tubular proteinuria suggest proximal dysfunction. Review drugs, toxins, monoclonal proteins, and inherited causes; replace measured deficits and treat the cause.

## Worked localization

Hypokalemic alkalosis with low magnesium, low urine calcium, and normal BP suggests a distal salt-wasting pattern. Review thiazide use first, then pursue specialist evaluation for Gitelman syndrome when appropriate.

## Source lessons

- [tubular disorders student handout](https://urinenephrology.org/student-resources/pediatric/tubular-disorders-student-handout.html)
- [chapter 16 tubulointerstitial](https://urinenephrology.org/nephrology-textbook/chapters/chapter-16-tubulointerstitial.html)

Read alongside the full lessons; the findings and decisions shown here require the stated clinical context.