Kidney organ test biochemistry answer
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
Reading File
Reading File
Finding Sources
Finding Sources
Finding Sources
Finding Sources
Reading File
Finding Sources
I now have rich information from multiple authoritative sources. Let me compile the comprehensive answer.
Kidney Function Tests - Biochemistry
Sources: Harper's Illustrated Biochemistry 32nd Ed | Tietz Textbook of Laboratory Medicine 7th Ed | Goldman-Cecil Medicine | Harriet Lane Handbook 23rd Ed | Barash's Clinical Anesthesia 9th Ed | Morgan & Mikhail's Clinical Anesthesiology 7th Ed
Overview
Kidney function tests assess the ability of the kidneys to filter waste, concentrate urine, and maintain fluid/electrolyte balance. They are grouped into:
- Tests of Glomerular Function
- Tests of Tubular Function
- Urinalysis
1. URINALYSIS (First-Line Assessment)
A complete urinalysis is the first step in evaluating kidney function. It includes:
| Test | Normal Value | Clinical Significance |
|---|---|---|
| Appearance | Colorless to pale amber, clear | Dark/turbid = concentrated urine, infection, proteinuria |
| Specific gravity | 1.003 - 1.030 | Reflects concentrating ability; isosthenuria (1.010) = tubular disease |
| pH | 4.5 - 8 (avg 5-6) | Alkaline urine = UTI with urea-splitting organisms |
| Protein | < 150 mg/24 hr (< 30 mg albumin) | Proteinuria = glomerular disease |
| Glucose | Absent | Glycosuria without hyperglycemia = tubular dysfunction (Fanconi) |
| Ketones | Absent | Seen in DKA, starvation |
| Blood/Hb | Absent | Hematuria, hemoglobinuria |
| Nitrite | Absent | Gram-negative bacterial infection |
| Bilirubin/Urobilinogen | Absent / Trace | Liver disease indicators |
Dipstick ("Reagent Strip") Testing - a rapid point-of-care method. Automated readers reduce inter-operator variability. Timing of color change must be carefully monitored (30 seconds to 2 minutes depending on analyte).
2. TESTS OF GLOMERULAR FUNCTION
A. Serum Creatinine
- Source: Creatinine is the end product of creatine phosphate metabolism in skeletal muscle
- Normal values: ~0.6-1.2 mg/dL (men); ~0.5-1.0 mg/dL (women)
- A significant rise in serum creatinine only occurs after ~50% decline in GFR - making it a test of poor sensitivity for early renal disease
- Despite this limitation, it is the most widely used and practical filtration marker
- Why better than urea: Blood creatinine is not significantly affected by non-renal factors, making it a more specific indicator of renal function
(Harper's Biochemistry 32nd Ed, Tests of Kidney Function)
B. Blood Urea Nitrogen (BUN)
- Source: Urea is produced in the liver as the end product of protein catabolism (urea cycle)
- Normal BUN: ~10-20 mg/dL
- BUN:Creatinine ratio: Normally 10:1 to 20:1 (in mg/dL)
- Ratio > 20:1 → Prerenal azotemia (dehydration, cardiac failure, GI bleed)
- Ratio < 10:1 → Intrinsic renal disease, low protein intake, liver disease
- Limitations of BUN: Affected by many non-renal factors:
- Elevated by: high protein diet, GI bleeding, fever, corticosteroids, dehydration
- Decreased by: malnutrition, liver disease, SIADH
(Goldman-Cecil Medicine; Comprehensive Clinical Nephrology 7th Ed)
C. Serum Uric Acid
- End product of purine catabolism
- Elevated in renal failure and contributes to CKD progression
- Higher serum uric acid is a risk factor for CKD progression
- Can form uric acid crystals in acidic urine (visible on urinalysis)
D. Creatinine Clearance (CrCl) - Estimates GFR
Formula:
CrCl (mL/min) = [U × V] / P
Where:
-
U = urine creatinine concentration (mg/dL)
-
V = urine volume per minute (total volume ÷ 1440 min for 24-hr collection)
-
P = plasma creatinine concentration (mg/dL)
-
Normal GFR: ~120 ± 25 mL/min (men); ~95 ± 20 mL/min (women)
-
GFR declines ~10% per decade after age 30
-
CrCl slightly overestimates GFR because creatinine is also secreted by tubules
-
In children, corrected to body surface area: CrCl (mL/min/1.73 m²)
(Barash's Clinical Anesthesia 9e; Morgan & Mikhail's Clinical Anesthesiology 7e)
E. Estimated GFR (eGFR) - Predictive Formulas
Three commonly used equations:
| Formula | Variables Used | Best For |
|---|---|---|
| Cockcroft-Gault | Age, weight, sex, serum creatinine | Drug dosing |
| MDRD | Age, sex, race, serum creatinine | Stable CKD |
| CKD-EPI | Age, sex, race, serum creatinine | Higher GFR values; most accurate |
Cockcroft-Gault formula:
eGFR (mL/min) = (140 - age) × weight(kg) / (Cr × 72) × 0.85 (for females)
- MDRD underestimates GFR in patients with GFR > 60 mL/min/m²
- CKD-EPI is more accurate across a wider range and is currently preferred
(Barash's Clinical Anesthesia 9e)
F. Gold Standard - Inulin Clearance
- Inulin is an exogenous polysaccharide that is:
- Freely filtered at the glomerulus
- Neither reabsorbed nor secreted by tubules
- Excreted only in urine
- Requires intravenous infusion at constant rate - not practical for routine use
- Other gold standards: ⁵¹Cr-EDTA or ⁹⁹Tc-DTPA clearance (expensive, cumbersome)
G. Cystatin C
- A low-molecular-weight protein produced at a constant rate by all nucleated cells
- Accumulates in circulation with renal impairment
- Theoretical advantages over creatinine:
- Less affected by muscle mass, age, sex
- More sensitive early marker of GFR decline
- Not yet replaced creatinine in routine practice
(Miller's Anesthesia 10e; Brenner & Rector's The Kidney)
3. PROTEINURIA - Key Glomerular Marker
| Type | Amount | Significance |
|---|---|---|
| Normal | < 150 mg/24 hr total; < 30 mg albumin/24 hr | Below detection on routine tests |
| Microalbuminuria | 30-300 mg albumin/24 hr | Early marker of renal damage in diabetes mellitus |
| Macroalbuminuria/Proteinuria | > 300 mg/24 hr | Established nephropathy |
| Nephrotic-range proteinuria | > 3.5 g/24 hr | Nephrotic syndrome |
- Most common cause: loss of integrity of the glomerular basement membrane (glomerular proteinuria)
- Dominant protein in glomerular proteinuria = albumin
(Harper's Biochemistry 32nd Ed)
4. TESTS OF TUBULAR FUNCTION
A. Urine Concentrating Ability
- Tests vasopressin (ADH)-controlled water reabsorption in the distal tubule
- Water deprivation test: Urine osmolality should rise to > 800 mOsm/kg in normal kidneys
- Isosthenuria (specific gravity fixed at 1.010) = loss of concentrating/diluting ability = severe tubular damage
B. Urine Osmolality
- Reflects concentrating ability more accurately than specific gravity
- Normal urine osmolality: 50-1200 mOsm/kg (varies with hydration)
C. Tubular Maximum (Tm) Tests
- Measure maximum tubular reabsorptive/secretory capacity
- e.g., Tm for glucose = ~375 mg/min; renal threshold for glucose = ~180 mg/dL serum glucose
D. Fractional Excretion of Sodium (FeNa)
FeNa (%) = (urine Na × plasma Cr) / (plasma Na × urine Cr) × 100
| FeNa | Interpretation |
|---|---|
| < 1% | Prerenal azotemia (Na avidly reabsorbed) |
| > 2% | Intrinsic renal (ATN) or post-renal cause |
5. CKD CLASSIFICATION (Based on eGFR)
| Stage | GFR (mL/min/1.73 m²) | Description |
|---|---|---|
| G1 | ≥ 90 | Normal or high |
| G2 | 60-89 | Mildly decreased |
| G3a | 45-59 | Mild-moderate decrease |
| G3b | 30-44 | Moderate-severe decrease |
| G4 | 15-29 | Severely decreased |
| G5 | < 15 | Kidney failure (dialysis needed) |
- GFR < 60 mL/min = criteria for CKD
- GFR < 15 mL/min = uremic symptoms; may require dialysis
Quick Summary Table - Key Kidney Biochemistry Values
| Test | Normal Value | Significance if Elevated |
|---|---|---|
| Serum creatinine | 0.6-1.2 mg/dL (men) | Impaired GFR (rises after 50% loss of GFR) |
| BUN | 10-20 mg/dL | Azotemia; also elevated in dehydration, GI bleed |
| BUN:Creatinine ratio | 10:1 to 20:1 | >20 = prerenal; <10 = hepatic/malnutrition |
| GFR (normal) | 90-140 mL/min | <60 = CKD; <15 = kidney failure |
| Urine protein | <150 mg/24 hr | Proteinuria = glomerular disease |
| Microalbuminuria | 30-300 mg/24 hr | Early diabetic nephropathy |
| Urine specific gravity | 1.003-1.030 | Fixed at 1.010 = isosthenuria = tubular damage |
This is a shared conversation. Sign in to Orris to start your own chat.