5.4 Interpreting Biochemical Test Results

Interpreting biochemical test results is crucial for assessing nutritional status. These tests measure protein, iron, and vitamin levels in the body, providing insights into overall health. However, factors like inflammation, disease, and medications can affect results, making interpretation complex.

To accurately assess nutritional status, it's important to consider multiple markers and integrate them with clinical data. This approach helps overcome limitations of individual tests and provides a more comprehensive picture of a person's nutritional health. Monitoring changes over time can also reveal trends and responses to interventions.

Biochemical Tests for Nutritional Status

Protein Status Markers

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• Albumin common marker of protein status, levels below 3.5 g/dL indicate protein malnutrition
  • Long half-life of about 20 days reflects long-term protein intake
• Prealbumin (transthyretin) shorter half-life of 2-3 days, more sensitive indicator of acute changes in protein status compared to albumin
  • Levels below 15 mg/dL suggest protein malnutrition
• Transferrin iron-transport protein used to assess both protein status and iron deficiency anemia
  • Levels below 200 mg/dL may indicate protein malnutrition or iron deficiency
• Retinol-binding protein (RBP) carrier for vitamin A, half-life of 12 hours, very sensitive marker of short-term protein status
  • Levels below 3 mg/dL suggest protein malnutrition

Iron and Vitamin D Status Markers

• Hemoglobin and hematocrit used to assess iron status and diagnose anemia
  • Hemoglobin levels below 12 g/dL in women and 13 g/dL in men indicate anemia
  • Hematocrit below 36% in women and 41% in men indicate anemia
• Serum ferritin reflects total body iron stores
  • Levels below 12 ng/mL indicate iron deficiency
  • Ferritin is an acute-phase reactant that can be elevated in inflammation, masking iron deficiency
• Vitamin D status assessed using serum 25-hydroxyvitamin D (25(OH)D)
  • Levels below 20 ng/mL indicate deficiency
  • Levels between 21-29 ng/mL suggest insufficiency

Factors Influencing Biochemical Markers

Inflammatory and Disease States

• Inflammation elevates levels of acute-phase proteins (ferritin, C-reactive protein (CRP), alpha-1-acid glycoprotein (AGP))
  • Decreases levels of negative acute-phase proteins (albumin, prealbumin, transferrin)
  • Complicates interpretation of these markers in assessing nutritional status
• Liver disease impairs synthesis of proteins (albumin, prealbumin, transferrin)
  • Leads to decreased levels even in the absence of malnutrition
• Kidney disease results in increased losses of small proteins (prealbumin, RBP)
  • Leads to decreased levels that do not necessarily reflect nutritional status

Individual Factors and Confounders

• Hydration status affects concentration of biochemical markers
  • Dehydration leads to falsely elevated levels
  • Overhydration results in falsely decreased levels
• Medications interfere with absorption, metabolism, or excretion of nutrients, affecting biochemical markers
  • Proton pump inhibitors reduce iron absorption
  • Diuretics increase urinary losses of electrolytes and minerals
• Age, sex, and ethnicity influence reference ranges for certain biochemical markers
  • Requires the use of population-specific cutoffs for interpretation

Limitations of Biochemical Markers

Snapshot Nature and Non-Specificity

• Biochemical markers provide a snapshot of nutritional status at a single point in time
  • May not reflect long-term or overall nutritional status
• Many biochemical markers are not specific to nutritional status
  • Affected by non-nutritional factors (inflammation, liver or kidney disease, hydration status, medications)
  • Leads to potential misinterpretation
• Some nutrients (zinc, selenium) lack reliable or widely available biochemical markers
  • Limits the ability to assess status using biochemical tests alone

Sensitivity and Reference Range Issues

• Biochemical markers may not detect subclinical or early-stage nutrient deficiencies
  • Deficiencies may not have yet progressed to overt deficiency states
• Reference ranges for biochemical markers often based on healthy populations
  • May not be appropriate for individuals with certain diseases or conditions that alter nutrient requirements or metabolism

Integrating Biochemical Markers and Clinical Data

Comprehensive Assessment Strategies

• Use multiple biochemical markers reflecting different aspects of nutritional status (protein status, iron status, vitamin status)
  • Provides a more comprehensive assessment than relying on a single marker alone
• Compare results of biochemical markers with clinical signs and symptoms of nutrient deficiencies or excesses
  • Helps corroborate findings and guide interpretation of potentially conflicting or equivocal biochemical results
• Incorporate dietary intake data (food records, 24-hour recalls) to provide context for interpreting biochemical markers
  • Helps distinguish between dietary insufficiency and non-nutritional factors affecting biochemical results

Longitudinal Monitoring and Holistic Evaluation

• Consider medical history, medication use, and other clinical data to identify potential confounding factors
  • Guides selection of appropriate markers for an individual's unique circumstances
• Monitor changes in biochemical markers over time, in conjunction with clinical status and dietary intake
  • Provides a more dynamic assessment of nutritional status and response to interventions than a single set of biochemical results
• Integrate biochemical markers with anthropometric measurements, body composition assessment, and functional measures of nutritional status (grip strength, physical performance)
  • Provides a more holistic evaluation of an individual's nutritional health