Introduction:
Nonprotein Nitrogen (NPN) constituents refer to small nitrogen-containing compounds in blood plasma that are not part of proteins. These include urea, creatinine, uric acid, ammonia, and amino acids. Measurement of these compounds provides valuable insight into renal function and the efficiency of glomerular filtration and tubular excretion.
When the kidney’s ability to form or excrete urine is impaired, the concentrations of NPN substances in plasma increase significantly. Among these, urea and creatinine are the most clinically useful markers, while uric acid provides supplementary information.
Main NPN Constituents
- Urea – End product of protein metabolism.
- Creatinine – Derived from creatine phosphate in muscle tissue.
- Uric Acid – End product of purine metabolism.
Historically, total NPN was measured by estimating total nitrogen in serum, but modern clinical chemistry focuses on urea nitrogen and creatinine levels for assessing kidney function.
Creatinine and Creatinine Clearance
1. Origin and Significance
Creatinine is a waste product of muscle metabolism. It is formed from creatine phosphate in muscle and is excreted in urine as a by-product. The rate of creatinine production is constant and depends mainly on muscle mass. Since creatinine is filtered by the glomeruli and not reabsorbed by renal tubules, it serves as a reliable indicator of glomerular filtration rate (GFR).
- Normal daily excretion: 1–2 g/day (varies with muscle mass).
- Serum creatinine is unaffected by diet or exercise.
- Increased serum creatinine = reduced GFR = possible renal impairment.
2. Creatinine Clearance Test
Creatinine clearance is the most sensitive chemical test for estimating glomerular filtration rate (GFR). It measures how effectively the kidneys remove creatinine from the blood.
Formula:
Creatinine Clearance (C) = (U × V) / S
- U = Urine creatinine concentration
- S = Serum creatinine concentration
- V = Urine volume excreted per minute
The result is expressed in mL/min. Since creatinine is freely filtered and minimally secreted, its clearance approximates the GFR.
Normal Values:
- Men: 95–140 mL/min
- Women: 90–130 mL/min
- Children: 55–85 mL/min/1.73 m²
Interpretation:
- Decreased Clearance: Indicates reduced GFR due to kidney disease, glomerular damage, or renal blood flow reduction.
- Increased Clearance: Usually due to collection errors or incomplete timing; not clinically significant.
3. Serum Creatinine Estimation
Principle:
Creatinine reacts with alkaline picrate to form a reddish complex — the Jaffe reaction. The color intensity is proportional to the creatinine concentration and is measured spectrophotometrically.
However, noncreatinine chromogens (such as glucose, ketones, and ascorbic acid) can interfere. Enzymatic methods or specific adsorbents (like Lloyd’s reagent) are used to obtain accurate readings.
Increased Serum Creatinine:
- Renal disease or obstruction.
- Congestive heart failure.
- Dehydration or shock.
- Diabetes mellitus or excessive diuretic use.
Decreased Serum Creatinine:
Usually has no clinical significance.
Urea and Urea Clearance
1. Origin and Role
Urea is the end product of protein catabolism. It is produced in the liver and excreted by the kidneys. Measuring urea nitrogen (urea-N) helps assess renal excretory function and protein metabolism status.
2. Urea Clearance Test
Urea clearance is defined as the volume of plasma cleared of urea per minute. The formula is:
Urea Clearance = (U × V) / P
- U = Urinary urea (mg/dL)
- P = Plasma urea (mg/dL)
- V = Urine volume (mL/min)
Normal Value:
75 mL/minute
Interpretation:
- 40–70 mL/min: Mild impairment
- 20–40 mL/min: Moderate impairment
- <20 mL/min: Severe impairment
Limitations:
Unlike creatinine, urea is reabsorbed (40–60%) by renal tubules, making urea clearance less accurate for GFR estimation. It is also influenced by diet, hydration, and infection.
3. Serum Urea Nitrogen Estimation
Principle:
The urease method is used. Urease hydrolyzes urea into ammonia (NH₃) and carbon dioxide (CO₂). The liberated ammonia reacts with phenol and sodium hypochlorite to form a blue indophenol dye (Berthelot reaction), measurable spectrophotometrically at 630 nm.
Normal Values:
- 8–18 mg/dL (or 2.9–6.4 mmol/L)
Increased Urea Levels:
- High protein diet or steroid therapy
- Stressful conditions (cortisol release)
- Renal disease or urinary obstruction
Decreased Urea Levels:
- Late pregnancy
- Severe protein deficiency or starvation
Uric Acid
1. Source and Importance
Uric acid is a purine metabolism end product that circulates as sodium urate. It is excreted by the kidneys, but if production exceeds excretion, uric acid crystals may deposit in joints and tissues, leading to gout or renal complications.
Clinical Relevance:
- Elevated uric acid: Seen in gout, renal disease, leukemia, pregnancy toxemia, and post-cancer therapy.
- Decreased uric acid: May occur after corticosteroid therapy or with uricosuric drugs (probenecid, allopurinol).
2. Serum Uric Acid Estimation
Principle:
Traditionally, uric acid reduces phosphotungstic acid to a blue phosphotungstate complex, measured at 700 nm. However, the enzymatic method using uricase is more specific. Uricase oxidizes urate to allantoin, hydrogen peroxide, and carbon dioxide, allowing accurate quantification.
Reference Values:
- Men: 3.5–7.5 mg/dL
- Women: 2.5–6.5 mg/dL
- Children: 2.0–5.5 mg/dL
3. Urine Uric Acid Estimation
The same procedure used for serum uric acid is applied to urine after appropriate dilution (usually 1:10). Average urinary uric acid excretion ranges between 250–750 mg/24 hours, depending on diet and metabolism.
Summary Table: Major NPN Constituents
| Constituent | Source | Clinical Importance |
|---|---|---|
| Creatinine | Muscle metabolism (Creatine phosphate) | Indicator of glomerular filtration rate (GFR) |
| Urea | Protein catabolism (Liver) | Reflects renal excretion and protein intake |
| Uric Acid | Purine metabolism | Marker for gout, renal stones, and tissue breakdown |
Detailed Notes:
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