Skip to main content

Chemistry

Procedures

This section covers general endocrinology testing principles, precautions, specimen handling, troubleshooting, interfering substances, and stimulation/suppression testing

General Principles of Endocrine Testing

  • Purpose: To measure hormone levels and assess endocrine gland function for diagnosis and management of endocrine disorders
  • Types of Assays
    • Immunoassays: Based on antibody-antigen interactions (e.g., ELISA, CLIA, RIA)
    • Fluorescence Assays: Measure the intensity of emitted light from fluorescently labeled hormones
    • Liquid Chromatography-Mass Spectrometry (LC-MS): Separates and quantifies hormones based on their mass-to-charge ratio

General Precautions

  • Patient Preparation
    • Fasting: Required for some tests to minimize the effect of food intake on hormone levels
    • Medications: Some drugs can interfere with hormone tests; the physician should advise on whether to withhold medications
    • Timing: Specimen collection should be timed appropriately due to diurnal variations or pulsatile secretion of hormones
    • Stress: Minimize patient stress during specimen collection, as stress can affect hormone levels
    • Menstrual Cycle: For women, the phase of the menstrual cycle can affect sex hormone levels; specimens should be collected at the specified phase
  • Specimen Collection
    • Use appropriate collection tubes for each hormone test
    • Follow the correct order of draw when collecting multiple tubes to prevent cross-contamination
    • Use proper venipuncture technique to minimize hemolysis and contamination
    • Label specimens immediately and accurately to prevent misidentification
    • Collect the correct volume of blood to ensure adequate sample for testing
  • Specimen Processing
    • Centrifuge specimens promptly after collection to separate serum or plasma from cells
    • Separate serum or plasma from cells within the specified time frame to prevent hormone degradation
    • Aliquot specimens into appropriate containers to prevent contamination and allow for multiple tests
    • Store specimens at the appropriate temperature to preserve hormone stability
    • Thaw specimens properly before testing to prevent degradation of hormones

Troubleshooting

  • High Hormone Results
    • Pre-analytical: Patient not fasting, specimen collected at the wrong time, medication interference, stress during collection, improper collection tube, delayed processing, contamination of specimen
    • Analytical: Incorrect calibration, reagent deterioration, interfering substances, cross-reactivity, non-specific binding
  • Low Hormone Results
    • Pre-analytical: Improper collection tube, delayed processing, improper storage, medication interference
    • Analytical: Incorrect calibration, reagent deterioration, interfering substances, assay drift
  • Inconsistent Results
    • Pre-analytical: Patient variability, specimen handling errors
    • Analytical: Assay variability, interfering substances, cross-reactivity
    • Post-analytical: Transcription errors, reporting errors
  • Quality Control Failures
    • Pre-analytical: QC material improperly stored, QC material expired
    • Analytical: Incorrect calibration, reagent deterioration, instrument malfunction
    • Action: Investigate the cause of the failure and take corrective action

Interfering Substances

  • Heterophile Antibodies: Bind to assay antibodies, causing falsely high or low results; use blocking reagents to mitigate
  • Biotin: High levels can interfere with biotin-streptavidin assays; ask patients to discontinue biotin supplements
  • Hemolysis: Releases intracellular components that can interfere with hormone assays; avoid hemolysis during collection and processing
  • Lipemia: Turbidity from high lipid concentrations can interfere with spectrophotometric assays; use lipemia clearing techniques
  • Bilirubin: High bilirubin levels can interfere with spectrophotometric assays; use methods less susceptible to bilirubin interference
  • Drugs: Certain drugs can directly affect hormone levels or interfere with hormone assays; be aware of patient’s medication list
  • Cross-Reactivity: Assay antibodies may cross-react with structurally similar compounds; use highly specific antibodies
  • Non-Specific Binding: Non-specific binding of assay components to the solid phase can lead to inaccurate results; use blocking reagents

Stimulation/Suppression Tests

  • Purpose: To evaluate the functional reserve of an endocrine gland or assess the autonomy of hormone production
  • General Procedure
    • Baseline sample collection
    • Administration of stimulatory or suppressive agent
    • Serial sample collection
    • Interpretation of hormone response
  • Common Stimulation Tests
    • ACTH Stimulation Test (Cosyntropin Test): Assesses adrenal gland function
    • Insulin Tolerance Test (ITT): Assesses growth hormone and ACTH reserve; requires close medical supervision due to the risk of severe hypoglycemia
    • TRH Stimulation Test: Assesses pituitary TSH reserve
  • Common Suppression Tests
    • Dexamethasone Suppression Test (DST): Diagnoses Cushing’s syndrome and differentiates its causes
    • Oral Glucose Suppression Test: Diagnoses acromegaly
  • General Precautions
    • Patient Selection: Ensure the patient is an appropriate candidate and there are no contraindications
    • Medication Review: Review the patient’s medication list
    • Informed Consent: Obtain informed consent from the patient
    • Adverse Reactions: Be prepared to manage potential adverse reactions
    • Standardized Procedures: Follow standardized procedures for test administration, sample collection, and hormone measurement
    • Quality Control: Implement rigorous quality control procedures
    • Reference Ranges: Use appropriate reference ranges
    • Clinical Correlation: Interpret test results in the context of the patient’s clinical presentation

Key Terms

  • Pre-Analytical: Processes that occur before the sample is analyzed
  • Analytical: Processes involved in analyzing the sample
  • Post-Analytical: Processes that occur after the sample is analyzed
  • Interfering Substance: A substance that affects the accuracy of a test
  • Heterophile Antibodies: Antibodies that bind to multiple antigens
  • Cross-Reactivity: The ability of an antibody to bind to multiple antigens
  • Stimulation Test: A test used to assess hormone deficiency by stimulating hormone release
  • Suppression Test: A test used to assess hormone excess by suppressing hormone production
  • Dynamic Testing: Assessing endocrine function by measuring the response to a challenge
  • Baseline Sample: A blood sample collected prior to the administration of a stimulatory or suppressive agent
  • Reference Range: The range of values found in healthy individuals