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# Tests of renal tubular function

Creatinine and GFR measure filtration; these tests ask whether the tubules can concentrate urine, acidify it and reabsorb what the body needs.

Updated Nov 12, 2026

## Key facts

- Concentrating ability: urine osmolality after fluid restriction; a kidney that cannot concentrate passes dilute urine despite dehydration.
- Water deprivation test with desmopressin separates central diabetes insipidus (urine concentrates after desmopressin) from nephrogenic diabetes insipidus (it does not).
- Acidification test: after an acid load, urine pH should fall below about 5.5; in distal renal tubular acidosis it does not.
- Proximal tubule damage shows as glucose in urine with normal blood glucose, phosphate and bicarbonate loss, aminoaciduria and low-molecular-weight proteins (β₂-microglobulin).
- Fractional excretion of sodium (FENa) below 1 % suggests a prerenal cause of acute kidney injury; above about 2 % suggests tubular damage.

## In detail

- A urine specific gravity fixed near 1.010, whatever the fluid intake, means the kidney neither concentrates nor dilutes (advanced chronic kidney disease).
- Renal threshold and transport maximum: glucose appears in urine when the filtered load exceeds the tubules' maximum reabsorption, normally when blood glucose rises well above the normal fasting range.
- After diuretics, fractional excretion of urea is more reliable than FENa for prerenal states.
- Urea clearance was used to estimate filtration but is affected by urine flow and tubular reabsorption, so creatinine-based estimates replaced it.

**Also called**
Urine concentration and acidification tests, Fractional excretion of sodium

## Clinical notes

- Hypokalaemic, normal anion gap acidosis with a urine pH that stays high suggests distal renal tubular acidosis.
- Glycosuria with normal blood glucose suggests renal glycosuria, Fanconi syndrome or SGLT2-inhibitor treatment.

## Test yourself

Think of the answer, then tap the question to see it.

## How does the water deprivation test distinguish central from nephrogenic diabetes insipidus?

After desmopressin, urine concentrates in central diabetes insipidus but not in nephrogenic, where the kidney does not respond to ADH.

## What does a FENa below 1 % suggest in acute kidney injury?

A prerenal cause: the tubules are working and avidly retaining sodium.

## What does failure to lower urine pH after an acid load indicate?

Distal renal tubular acidosis: the collecting duct cannot secrete enough H⁺.

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## Connections

## Related conditions 3

- [Renal tubular acidosis](https://durgical.com/en/conditions/renal-tubular-acidosis/)
- [Diabetes insipidus (central and nephrogenic)](https://durgical.com/en/conditions/diabetes-insipidus/)
- [Fanconi syndrome](https://durgical.com/en/conditions/fanconi-syndrome/)

## Related 3

- [Creatinine and estimated glomerular filtration rate (eGFR)](https://durgical.com/en/biochemistry/topics/creatinine-and-estimated-glomerular-filtration-rate/)
- [Anion gap and delta ratio](https://durgical.com/en/biochemistry/topics/anion-gap-and-delta-ratio/)
- [Acid–base interpretation (step by step)](https://durgical.com/en/biochemistry/topics/acid-base-interpretation/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Urine concentration test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003608.htm) (MedlinePlus, US National Library of Medicine, 2026); [Acid loading test (pH): MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003615.htm) (MedlinePlus, US National Library of Medicine, 2026); [Osmolality Tests: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/osmolality-tests/) (MedlinePlus, US National Library of Medicine, 2026).

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