Renal function and renal function test.
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Renal Function and Renal Function Tests
1. Overview of Renal Function
The kidneys perform several essential physiologic roles. Specialized epithelial cells of the nephrons and collecting systems produce urine through three core processes:
- Filtration - water and solutes leave the vascular space and enter the nephron lumen at the glomerulus
- Tubular secretion - substances move from tubular epithelial cells into the lumen (from interstitium/capillaries)
- Tubular reabsorption - substances move from the tubular lumen back across the epithelium into the interstitium and capillaries
Along the nephron, the filtrate receives secreted molecules and has others reabsorbed before entering the collecting system as urine for excretion.
Beyond urine formation, the kidneys also:
- Regulate fluid and electrolyte balance
- Regulate acid-base homeostasis
- Produce erythropoietin (red cell production)
- Produce renin (blood pressure regulation)
- Activate vitamin D (1,25-dihydroxycholecalciferol)
- Excrete metabolic waste products (urea, creatinine, uric acid)
- Junqueira's Basic Histology, 17e; Barash's Clinical Anesthesia, 9e
2. Glomerular Filtration Rate (GFR)
GFR is the plasma volume filtered per unit time by the kidneys. It is the single most important measure of kidney function.
Normal Values:
| Age/Sex | Mean GFR (mL/min/1.73 m²) |
|---|---|
| 1 week | 41 ± 15 |
| 2-8 weeks | 66 ± 25 |
| >8 weeks | 96 ± 22 |
| 2-12 years | 133 ± 27 |
| Adults (general) | 90-140 mL/min |
- GFR declines ~10% per decade after age 30
- GFR is approximately 10 mL/min higher in men than women
- GFR <60 mL/min = criteria for Chronic Kidney Disease (CKD)
- GFR <15 mL/min = uremic symptoms; may require dialysis
- Harriet Lane Handbook, 23e; Barash's Clinical Anesthesia, 9e
3. Renal Function Tests
A. Serum Creatinine
Creatinine is the most commonly used kidney filtration marker. It is a byproduct of muscle metabolism (creatine phosphate breakdown).
Properties making it useful (but imperfect):
- Relatively steady supply from muscle metabolism
- Freely filtered at the glomerulus
- Modest tubular secretion (slight overestimates CrCl)
- Cheap and easy to measure
Normal serum creatinine:
- Males ≥18 years: ~0.7-1.3 mg/dL
- Females: slightly lower (less muscle mass)
Limitations: Creatinine rises only after ~50% of nephron function is lost (insensitive early marker). Also influenced by muscle mass, diet, age, and sex.
- Barash's Clinical Anesthesia, 9e; Morgan & Mikhail's Clinical Anesthesiology, 7e
B. Blood Urea Nitrogen (BUN)
BUN is a product of protein catabolism. The normal BUN-to-creatinine ratio is approximately 10:1 to 20:1 (mg/dL).
BUN:Creatinine ratio interpretation:
| Ratio | Interpretation |
|---|---|
| 10:1 to 20:1 | Normal |
| >20:1 | Prerenal azotemia (volume depletion, heart failure, cirrhosis, nephrotic syndrome, obstruction, high protein catabolism) |
| <10:1 | Malnutrition, liver disease (reduced urea synthesis), low protein intake |
Low renal tubular flow enhances urea reabsorption but does not affect creatinine excretion - this is why the ratio rises in prerenal states.
- Comprehensive Clinical Nephrology, 7e; Morgan & Mikhail, 7e; Goldman-Cecil Medicine
C. Creatinine Clearance (CrCl) - Measurement of GFR
1. Direct Measurement (24-hour urine collection)
Formula:
CrCl (mL/min) = [Ucr (mg/dL) × V (mL)] / [Pcr (mg/dL) × time (min)]
Where:
- U_cr = urine creatinine concentration
- V = total urine volume collected
- P_cr = plasma creatinine
- Time = collection duration in minutes (24 hr = 1440 min)
For body surface area correction:
CCr (mL/min/1.73 m²) = [U × (V/P)] × 1.73/BSA
In stable critically ill patients, 2-hour urine collections are sufficient.
2. Estimated GFR Formulas (from single serum creatinine)
Cockcroft-Gault Equation (estimates CrCl):
CrCl = [(140 - age) × weight (kg)] / [72 × Scr (mg/dL)]
× 0.85 for females
- Uses age, sex, weight, serum creatinine
- One of the oldest and most widely used formulas
MDRD (Modification of Diet in Renal Disease) Equation:
GFR = 186 × (Scr)^-1.154 × (age)^-0.203
× 0.742 for females
× 1.210 for Black patients (older version)
- Most applicable for stable CKD patients
- Tends to underestimate GFR when GFR >60 mL/min/m²
CKD-EPI 2021 Equation (currently recommended by NKF-ASN):
- Most accurate estimation when compared to gold standard
- No longer includes race as a variable (updated in 2021)
- The National Kidney Foundation and American Society of Nephrology task force now recommend this equation
CrCl Severity Scale (Cockcroft-Gault):
| CrCl | Interpretation |
|---|---|
| 74-160 mL/min | Normal |
| 40-60 mL/min | Mild renal impairment |
| 15-40 mL/min | Moderate renal impairment |
| <15 mL/min | Severe renal impairment (dialysis indication) |
- Barash's Clinical Anesthesia, 9e; Roberts & Hedges' Clinical Procedures in Emergency, p.1731; Harriet Lane Handbook, 23e
3. Gold Standard Methods (research use)
Ideal substance for GFR measurement must be: freely filtered, not reabsorbed, not secreted, not metabolized, freely available.
- Inulin clearance - true gold standard, cumbersome
- ⁵¹Cr-EDTA clearance - radiolabeled, not routine
- ⁹⁹Tc-DTPA clearance - nuclear medicine scan
- Iothalamate clearance
These are expensive and impractical for clinical use.
D. Cystatin C
An emerging alternative filtration marker. More sensitive than creatinine, especially at higher GFR values and in patients with extremes of body habitus. Not yet universally available. The NKF-ASN task force recommends its use in conjunction with creatinine to improve GFR estimation accuracy.
- Barash's Clinical Anesthesia, 9e
E. Fractional Excretion of Sodium (FENa)
Used to differentiate causes of Acute Kidney Injury (AKI).
Formula:
FENa (%) = (SCr × UNa) / (SNa × UCr) × 100
Where: S = serum, U = urine, Cr = creatinine, Na = sodium
Interpretation in AKI:
| FENa | Interpretation |
|---|---|
| <1% | Prerenal (kidneys conserving Na appropriately - hypovolemia, reduced perfusion) |
| 1-2% | Borderline/overlapping |
| >2% | Intrinsic renal disease (ATN - salt-wasting by damaged nephrons) |
| >4% | Postrenal (obstruction) |
Clinical Example: A 65-year-old male collapses in heat. SCr = 2.0, SNa = 150, UNa = 18, UCr = 30.
FENa = (2.0 × 18)/(150 × 30) × 100 = 0.8% → Prerenal (hypovolemia).
- Roberts & Hedges' Clinical Procedures in Emergency; Comprehensive Clinical Nephrology, 7e
F. Urinalysis (UA)
A comprehensive urinalysis evaluates multiple aspects of renal function:
| Parameter | Normal | Clinical Significance |
|---|---|---|
| Appearance | Colorless to amber | Red/orange: myoglobin, hemoglobin, drugs (rifampin, propofol) |
| Specific Gravity | 1.003 - 1.030 | 1.010 = isosthenuria (= plasma osmolality). >1.018 after overnight fast = adequate concentrating ability. Low SG with high serum osmolality = diabetes insipidus |
| pH | 4.5 - 8 (avg 5-6) | pH >7 in systemic acidosis = renal tubular acidosis (RTA) |
| Protein | Negative/trace | >150 mg/24h = significant. Glomerular disease can cause heavy proteinuria (nephrotic: >3.5 g/day). Confirm with urine protein:creatinine ratio or 24-hr collection |
| Glucose | Negative | Glucosuria when blood glucose >160-180 mg/dL. Also: proximal RTA, SGLT2 inhibitors, pregnancy |
| Ketones | Negative-trace | DM, starvation, inborn errors of metabolism |
| Nitrite | Negative | Specific (90-100%) but not sensitive (15-82%) for Gram-negative UTI |
| Leukocyte esterase | Negative | Pyuria; sensitive (67-84%) for UTI |
| Hemoglobin/RBCs | <5 RBC/hpf | Hematuria (tumor, stones, infection, trauma). Dysmorphic RBCs = glomerular origin. False-positive: myoglobin (rhabdomyolysis) |
| WBCs | <5 WBC/hpf | Infection, sterile pyuria, inflammatory disorders |
| Casts | None | Hyaline (normal, dehydration); RBC casts = glomerulonephritis; WBC casts = pyelonephritis/interstitial nephritis; granular/waxy = advanced disease |
- Harriet Lane Handbook, 23e; Morgan & Mikhail, 7e
G. Urine Osmolality and Specific Gravity
- Urine osmolality elevated in hypovolemia (kidneys concentrating urine)
- Low urine sodium (<20 mEq/L) in volume depletion
- Specific gravity 1.010 ≈ 290 mOsm/kg (isosmotic with plasma)
- Urine chloride is a better index than sodium in volume depletion with metabolic alkalosis (e.g., from vomiting)
- Comprehensive Clinical Nephrology, 7e
4. AKI Staging (KDIGO Criteria)
| Stage | Serum Creatinine | Urine Output |
|---|---|---|
| 1 (Risk) | ≥1.5× baseline or ≥0.3 mg/dL rise in 48 hrs | <0.5 mL/kg/hr for ≥6 hrs |
| 2 (Injury) | ≥2× baseline | <0.5 mL/kg/hr for ≥12 hrs |
| 3 (Failure) | ≥3× baseline or ≥4.0 mg/dL or dialysis | <0.3 mL/kg/hr for ≥24 hrs or anuria ≥12 hrs |
Earlier RIFLE (ADQI) criteria graded AKI as: Risk (50% Cr rise), Injury (100%), Failure (200% or Cr >4 mg/dL).
- Barash's Clinical Anesthesia, 9e
5. Renal Function in Special Situations
Pregnancy
GFR increases to ~170 mL/min/1.73 m² by 20 weeks (due to increased cardiac output and renal blood flow). As a result:
- Serum creatinine, urea, and uric acid are lower than normal non-pregnant values
- Glucosuria up to 1000 mg/day may be present (normal)
- Proteinuria up to 300 mg/day may be present (normal)
- Near term, GFR returns toward pre-pregnancy values; uric acid rises dramatically due to increased tubular reabsorption
- Tietz Textbook of Laboratory Medicine, 7e
Aging
GFR declines approximately 10% per decade after age 30. Elderly patients may have significantly impaired GFR despite "normal" serum creatinine, because reduced muscle mass reduces creatinine production. This is why the Cockcroft-Gault formula (using age and weight) is essential in the elderly.
6. Summary Table: Key Tests at a Glance
| Test | What it Measures | Normal Value | Key Use |
|---|---|---|---|
| Serum Creatinine | GFR surrogate | 0.7-1.3 mg/dL (M) | CKD staging, drug dosing |
| BUN | Nitrogen waste/protein catabolism | 8-24 mg/dL | AKI, volume status |
| BUN:Cr ratio | Prerenal vs. intrinsic | 10:1-20:1 | AKI etiology |
| CrCl (24hr urine) | Direct GFR estimate | 74-160 mL/min | Accurate GFR in stable patient |
| eGFR (CKD-EPI) | Estimated GFR | >90 normal | CKD staging |
| FENa | Na handling | - | AKI etiology (<1% prerenal, >2% intrinsic) |
| Cystatin C | GFR (better at high GFR) | - | When muscle mass abnormal |
| Urinalysis | Multiple | See above | Screening, AKI workup |
| Urine osmolality | Concentrating ability | 50-1200 mOsm/kg | Volume status, DI |
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