explain renal function test

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Renal Function Tests (RFTs)

Renal function tests are a collection of blood and urine investigations used to assess how well the kidneys perform their core jobs: filtering waste from blood, regulating fluid and electrolyte balance, and maintaining acid-base homeostasis. In nearly all kidney diseases, impaired function is due to a reduced number of functioning nephrons rather than failure of individual nephrons - so these tests essentially quantify how many functioning nephrons remain.

Tietz Textbook of Laboratory Medicine, 7th Ed.
Barash, Cullen & Stoelting's Clinical Anesthesia, 9th Ed.
Textbook of Family Medicine, 9th Ed.

1. Urinalysis (UA)

Urinalysis is typically the first step in evaluating kidney disease. It has two parts:

A. Urine Dipstick (Reagent Strip)

A strip impregnated with chemical reagents that changes color on contact with urine. It tests for multiple analytes simultaneously:

Parameter	Normal	Significance
Protein/Albumin	Negative / trace	Positive = glomerular damage, nephrotic syndrome, or tubular injury
Glucose	Negative	Positive = diabetes or tubular reabsorption defect
Blood/Hb	Negative	Positive = hematuria, hemolysis, myoglobinuria
Nitrite	Negative	Positive = bacterial infection (gram-negative organisms)
Leukocyte esterase	Negative	Positive = pyuria, UTI, interstitial nephritis
pH	4.5-8.0	Helps assess renal tubular acidosis, stone risk
Specific Gravity	1.001-1.035	Reflects concentrating ability; correlates with osmolality
Bilirubin / Urobilinogen	Negative	Reflects hepatic/biliary disease
Ketones	Negative	Diabetic ketoacidosis, starvation

Note: Specific gravity is measured via a polyelectrolyte pH-shift method on the dipstick. Glucose and protein can artificially elevate specific gravity readings, so osmolality is preferred for formal assessment of concentrating ability.

B. Urine Microscopy

Centrifuged urine sediment is examined for:

Red blood cells (RBCs): Hematuria; dysmorphic RBCs suggest glomerulonephritis
White blood cells (WBCs): Infection or interstitial nephritis
Casts: Cylindrical structures made of Tamm-Horsfall glycoprotein (uromodulin)
- Hyaline casts: normal in small numbers
- RBC casts: glomerulonephritis (active urinary sediment)
- WBC casts: pyelonephritis or interstitial nephritis
- Granular/muddy brown casts: acute tubular necrosis (ATN)
Epithelial cells, fat droplets, bacteria

A urine sample with RBC casts, WBC casts, and proteinuria is called an "active urinary sediment" - a hallmark of inflammatory glomerular disease.

2. Urine Protein / Proteinuria Assessment

Proteinuria is a key marker of kidney disease severity. Three types exist:

Type	Pattern	Cause
Glomerular	Predominantly albumin	Glomerulonephritis, nephrotic syndrome, diabetic nephropathy
Tubular	Predominantly low-MW proteins (α1, α2, β-globulins)	Tubular injury, Fanconi syndrome
Overflow	Light chains, myoglobin	Myeloma, rhabdomyolysis

Measurement methods:

Dipstick: Detects albumin only; sensitivity threshold ~300 mg/L
Spot urine protein-to-creatinine ratio (PCR): Convenient; avoids 24-hour collection
Spot albumin-to-creatinine ratio (ACR): Preferred for detecting early diabetic nephropathy
24-hour urine protein collection: Still used in many programs for exact quantification
Microalbuminuria: Albumin levels below dipstick detection (30-300 mg/day); early sign of diabetic/hypertensive nephropathy

3. Blood Urea Nitrogen (BUN)

Normal range: 7-18 mg/dL

Urea is a product of protein/amino acid catabolism in the liver, filtered freely by glomeruli, and partially reabsorbed by tubules.
Elevated BUN is not specific for intrinsic renal disease - it can also rise with: hypovolemia, high protein intake, corticosteroid use, hypercatabolism, and gastrointestinal bleeding.
In volume depletion, urea is reabsorbed proportionally more than creatinine, causing a disproportionate BUN rise.
BUN can be falsely low in severe liver disease, malnutrition, and SIADH.

BUN-to-Creatinine Ratio

Ratio	Interpretation
~10:1	Intrinsic (renal) cause
>20:1	Pre-renal (dehydration, poor perfusion) or post-renal (obstruction)

4. Serum Creatinine

Normal range: ~0.6-1.2 mg/dL (men); ~0.5-1.0 mg/dL (women)

Creatinine is a breakdown product of creatine phosphate from muscle metabolism. It is:

Freely filtered at the glomerulus
Not reabsorbed
Modestly secreted by tubules (~5-10% at normal GFR; this proportion increases as GFR falls)

Key concept - the parabolic relationship: Creatinine rises in a non-linear fashion as GFR declines. Early in kidney disease, large drops in GFR produce only small rises in creatinine. At low GFRs, small further drops produce dramatic creatinine rises. This means creatinine is an insensitive early marker of kidney damage.

Factors that affect creatinine:

Muscle mass (athletes have higher baseline; elderly/cachectic have lower)
Age, sex, race, dietary meat intake
Drugs that block tubular creatinine secretion (cimetidine, trimethoprim, fenofibrate, salicylates) can raise creatinine without truly reducing GFR

5. Glomerular Filtration Rate (GFR)

GFR is the volume of plasma filtered by all glomeruli per unit time. It is the single best overall index of kidney function.

Normal: 90-140 mL/min; GFR declines ~10% per decade after age 30; men ~10 mL/min higher than women.

Estimating GFR (eGFR)

Gold standard methods use exogenous markers (inulin, 51Cr-EDTA, 99Tc-DTPA clearance) but these are expensive and cumbersome.

Practical methods:

Creatinine clearance (CrCl) - from 24-hour urine:

CrCl (mL/min) = [U_cr (mg/dL) × V (mL/24h)] / [P_cr (mg/dL) × 1440 min]
MDRD equation (Modification of Diet in Renal Disease):
- Uses: age, serum creatinine, sex
- Reasonably accurate for eGFR < 60 mL/min; may underestimate at higher GFRs
- May be inaccurate in elderly, non-white, or healthy persons
CKD-EPI equation - more accurate across all GFR ranges; current standard for lab reporting

CKD Staging by eGFR

Stage	eGFR (mL/min/1.73m²)	Description
G1	≥90	Normal or high
G2	60-89	Mildly decreased
G3a	45-59	Mild-moderately decreased
G3b	30-44	Moderately-severely decreased
G4	15-29	Severely decreased
G5	<15	Kidney failure (may require dialysis)

A GFR < 60 mL/min meets criteria for CKD. Values < 15 mL/min are associated with uremic symptoms.

6. Cystatin C

Cystatin C is a low-molecular-weight protein produced at a constant rate by all nucleated cells. It is:

Freely filtered at the glomerulus
Not secreted by tubules
Completely reabsorbed and catabolized in proximal tubules (not excreted in urine)
Independent of muscle mass, sex, and diet

Advantages over creatinine:

More sensitive to early GFR changes (shorter half-life means faster response to GFR fluctuations)
Superior to creatinine as a GFR marker in elderly patients, cachectic patients, and those with low muscle mass
Combined cystatin C + creatinine equations (CKD-EPI 2021) are now more accurate than creatinine alone

7. Electrolytes

Electrolytes assessed as part of RFTs:

Electrolyte	Reference Range	Renal Significance
Sodium (Na+)	136-145 mEq/L	Diluting/concentrating defects; SIADH vs. DI
Potassium (K+)	3.5-5.0 mEq/L	Hyperkalemia in CKD/AKI due to reduced excretion
Bicarbonate (HCO3-)	22-29 mEq/L	Metabolic acidosis in CKD (tubular H+ excretion failure)
Chloride (Cl-)	98-106 mEq/L	Assessed with anion gap for acid-base interpretation
Calcium (Ca2+)	8.5-10.5 mg/dL	Low in CKD (impaired Vit D activation)
Phosphate (PO4)	2.5-4.5 mg/dL	High in CKD (reduced excretion)

8. Other Tests

Test	Use
Uric acid	Elevated in gout nephropathy; tumor lysis syndrome
Urine osmolality	Better than specific gravity for assessing concentrating ability; used in AKI workup
Urine sodium (FENa)	Fractional excretion of sodium; helps differentiate pre-renal from intrinsic AKI
24-hour urine creatinine	Validates adequacy of 24-hour collection
Beta-2 microglobulin	Marker of tubular function; elevated in tubular injury
NGAL, KIM-1, IL-18	Novel biomarkers of acute kidney injury; not yet routine

Summary: When Each Test is Most Useful

Test	Best for
Dipstick + microscopy	Screening, monitoring, infection, hematuria
Serum creatinine / eGFR	Overall GFR estimate, CKD staging, drug dosing
BUN / BUN:Cr ratio	Differentiating pre-renal vs. intrinsic renal vs. post-renal causes
Cystatin C	GFR estimation in patients with extremes of muscle mass or elderly
24-hour urine protein	Quantifying proteinuria in nephrotic syndrome
ACR	Early diabetic/hypertensive nephropathy screening
Electrolytes	Assessing complications of renal failure (hyperkalemia, acidosis)
Urine osmolality / FENa	AKI workup, concentrating ability

The classification of both CKD and AKI are based primarily on GFR measures, proteinuria/albuminuria, serum creatinine trends, and urine output - making RFTs the backbone of all nephrology diagnosis and management.

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