3.3 Biochemical Assessment & Lab Values

3.3 Biochemical Assessment & Lab Values

Overview of Biochemical Assessment

Biochemical assessment provides objective, quantitative data regarding nutritional status, organ function, metabolic control, and fluid-electrolyte balance. For the Nutrition and Dietetic Technician, Registered (NDTR), laboratory testing is vital for identifying subclinical nutrient deficits before physical symptoms emerge and for monitoring the efficacy of medical nutrition therapy (MNT).

Laboratory parameters are categorized into visceral protein markers, glycemic indicators, renal function tests, serum electrolytes, and hematological indices. Interpreting lab values requires understanding physiological half-lives, hydration status, and the confounding influence of systemic acute-phase inflammatory responses.

Visceral Protein Markers & Acute-Phase Reactants

Visceral proteins synthesized by the liver have historically been used to infer somatic protein status. However, modern clinical practice recognizes that visceral protein synthesis is heavily influenced by systemic inflammation, hepatic function, and hydration status.

Transport Protein Half-Lives & Clinical Significance

• Serum Albumin (Normal: 3.5–5.0 g/dL):
  • Half-Life: ~20 days.
  • Clinical Utility: Due to its large circulating pool and long half-life, albumin is a poor indicator of acute nutritional changes or recent protein intake. It serves primarily as a long-term marker and a strong predictor of chronic disease morbidity and mortality. Hypoalbuminemia (<3.5 g/dL) is frequently driven by inflammation, volume overload, liver disease, or nephrotic syndrome.
• Transferrin (Normal: 200–400 mg/dL):
  • Half-Life: ~8 days.
  • Clinical Utility: Transferrin transports iron in blood plasma. Its concentration increases during iron deficiency anemia (as the liver synthesizes more transferrin to capture iron) and decreases during protein-energy malnutrition, liver disease, and acute inflammation.
• Prealbumin / Transthyretin (Normal: 15–36 mg/dL):
  • Half-Life: ~2 days.
  • Clinical Utility: Prealbumin has a small circulating pool and rapid turnover, making it sensitive to recent acute changes in energy and protein intake. Values <10 mg/dL reflect severe nutritional risk or severe metabolic stress.

Impact of Systemic Inflammation & C-Reactive Protein (CRP)

During acute trauma, infection, major surgery, or critical illness, the liver prioritizes the synthesis of positive acute-phase reactants such as C-Reactive Protein (CRP). Simultaneously, the synthesis of negative acute-phase reactants (albumin, prealbumin, and transferrin) drops sharply.

• Clinical Rule: High CRP levels accompanied by low albumin/prealbumin indicate that visceral protein suppression is driven by systemic inflammation rather than isolated malnutrition. Visceral protein levels cannot be accurately used as nutritional markers until CRP normalizes.

Glycemic Parameters & Diabetes Diagnostics

Glycemic monitoring is critical for MNT in diabetes mellitus, metabolic syndrome, and critical care:

• Fasting Blood Glucose (FBG):
  • Normal: 70–99 mg/dL
  • Impaired / Prediabetes: 100–125 mg/dL
  • Diabetes Diagnostic: >=126 mg/dL (on two separate tests)
• Glycated Hemoglobin (HbA1c):
  • Normal: <5.7%
  • Prediabetes: 5.7%–6.4%
  • Diabetes Diagnostic: >=6.5%
  • Glycemic Goal for Diabetic Adults: <7.0% (American Diabetes Association target)
  • Pathophysiology: HbA1c measures the percentage of hemoglobin coated with glucose, reflecting average blood glucose control over the preceding 2 to 3 months (the ~120-day lifespan of red blood cells).

Electrolyte & Renal Function Reference Values

Electrolyte and renal panels assess hydration status, acid-base equilibrium, and kidney excretion capacity.

Serum Electrolyte Standards

• Sodium (Na+): 135–145 mEq/L (Primary extracellular cation; hypernatremia indicates dehydration; hyponatremia indicates fluid overload or sodium loss).
• Potassium (K+): 3.5–5.0 mEq/L (Primary intracellular cation; hyperkalemia risk in renal failure; hypokalemia risk with vomiting/diuretics).
• Chloride (Cl-): 96–106 mEq/L (Major extracellular anion).
• Bicarbonate / Total CO2: 22–29 mEq/L (Reflects acid-base balance and respiratory/renal compensation).
• Serum Calcium: 8.5–10.5 mg/dL (Corrected Calcium = Measured Calcium + 0.8 * [4.0 - Serum Albumin]).
• Serum Phosphorus: 2.5–4.5 mg/dL (Crucial parameter in refeeding syndrome; rapid drop indicates refeeding risk).
• Serum Magnesium: 1.5–2.5 mEq/L (Intracellular cofactor; depletion common in alcoholism and refeeding).

Renal Biomarkers & Azotemia

• Blood Urea Nitrogen (BUN): 7–20 mg/dL (End product of protein catabolism).
• Serum Creatinine: 0.6–1.2 mg/dL (Product of muscle creatine breakdown; highly specific to kidney filtration capacity).
• BUN:Creatinine Ratio:
  • Normal Ratio: 10:1 to 15:1.
  • >20:1 Ratio: Indicates prerenal azotemia (dehydration, inadequate renal perfusion, high protein intake, or GI bleeding).
  • <10:1 Ratio: Indicates intrinsic renal damage or low protein intake/liver disease.

Hematological Markers & Anemia Classification

• Hemoglobin (Hb): Males 13.8–17.2 g/dL; Females 12.1–15.1 g/dL.
• Hematocrit (Hct): Males 40.7%–50.3%; Females 36.1%–44.3%.
• Mean Corpuscular Volume (MCV): 80–100 fL
  • Microcytic Anemia (<80 fL): Iron deficiency anemia, thalassemia.
  • Macrocytic Anemia (>100 fL): Vitamin B12 (cobalamin) deficiency, Folate deficiency.
  • Normocytic Anemia (80–100 fL): Anemia of chronic disease, acute blood loss.

Comprehensive Laboratory Reference Standards Table