KIDNEY FUNCTION TESTS

excretory, regulatory, endocrine

What are the three main functions of the kidney?

Glomerular function tests
Tubular function tests
Urine concentration and dilution tests
Clearance tests
Tests for specific substances in urine

What are the categories of kidney function tests?

Glomerular Filtration Rate (GFR)

assesses the filtering capacity of the glomeruli; it is the best overall index of kidney function

Inulin (ideal but impractical)
Creatinine (most commonly used clinically)
Radioisotopes (e.g., ⁹⁹mTc-DTPA)

What substances are commonly used to measure GFR?

To assess the ability of renal tubules to reabsorb and secrete substances.

What is the purpose of tubular function tests?

Concentration test (e.g., specific gravity, osmolality)
Dilution test (e.g., water loading test)
Acidification test
Tests for tubular proteinuria (e.g., β2-microglobulin)

Name common tests for tubular function

water deprivation test

A test to evaluate the kidney's ability to concentrate urine; evaluates ADH function and tubular response.

Diabetes insipidus
Chronic renal disease
Pyelonephritis

What conditions cause impaired concentration?

renal clearance

The volume of plasma from which a substance is completely cleared by the kidneys per unit time.

It decreases proportionally with declining GFR but is less accurate due to reabsorption.

How is urea clearance affected in renal disease?

cystatin C

It is a sensitive marker for GFR estimation, independent of muscle mass, useful in early kidney disease detection.

BUN

measure of urea in blood

Prerenal: dehydration, shock
Renal: glomerulonephritis, acute tubular necrosis
Postrenal: obstruction (e.g., stones, tumors)

What causes increased BUN?

10:1

normal bun:creatinine ratio

> 20:1

Prerenal azotemia BUN:Creatinine ratio

~10:1

Intrinsic renal BUN:Creatinine ratio

variable

Postrenal BUN:Creatinine ratio

serum creatinine

A waste product from muscle metabolism; elevated levels indicate impaired kidney function.

Hematuria with dysmorphic RBCs
RBC casts
Proteinuria

What urinalysis findings suggest glomerular disease?

Granular and epithelial casts
Low specific gravity
Glucosuria without hyperglycemia

What findings indicate tubular disease?

glomerular proteinuria

increased permeability (e.g., nephrotic syndrome)

tubular proteinuria

defective reabsorption

overflow proteinuria

increased small proteins (e.g., multiple myeloma)

post-renal proteinuria

inflammation/infection

microalbuminuria

Early marker of diabetic nephropathy; urine albumin 30–300 mg/day

specific gravity

A measure of urine concentration

diabetes insipidus, renal failure

what causes low specific gravity

dehydration, SIADH

what causes high specific gravity?

β2-microglobulin

Marker of tubular dysfunction; increased levels indicate impaired reabsorption.

NGAL

Neutrophil gelatinase-associated lipocalin; early marker of acute kidney injury (AKI)

KIM-1

Kidney Injury Molecule-1; another early biomarker for AKI and tubular injury