diagnostic testing

Chapter 34: Diagnostic Testing

Introduction

• Test objective: To determine the necessity for additional tests.

• Author: Jonny Hawkins, 2010.

Normal Structure and Function of Blood Cells

Cell Types

• Erythrocytes (Red Blood Cells, or RBCs)

• Thrombocytes (Platelets)

• Leukocytes (White Blood Cells, or WBCs)

Overview of Blood Composition

• Average blood volume: Approximately 5 L.

  • Plasma: 3 L (Fluid portion)

  • Cells: 2 L (Types include erythrocytes, thrombocytes, and leukocytes)

Erythrocytes (RBCs)

• Function: Contain hemoglobin (Hgb), which is critical for oxygen transport to and carbon dioxide transport from tissues.

• Additional roles:

  • Maintaining normal acid-base balance.

  • Development occurs in the bone marrow.

Thrombocytes (Platelets)

• Role: Integral to blood clotting.

  • Mechanism: Clump together to form a plug when a blood vessel ruptures.

  • Formation: Also occurs in the bone marrow.

Leukocytes (WBCs)

• Function: Primarily involved in inflammatory and immune responses.

Types of White Blood Cells (WBCs)

Plasma

Composition of Plasma

• Key components include:

  • Electrolytes:

  • Sodium (Na+), Potassium (K+), Calcium (Ca++), Magnesium (Mg++), Chloride (Cl–), Bicarbonate (HCO3 –), Phosphate (HPO4 –)

  • Plasma Proteins:

  • Albumin: Major protein maintaining colloidal osmotic pressure in blood, crucial for fluid balance.

  • Fibrinogen: Facilitates coagulation, converting into fibrin threads in the presence of ionized calcium (Ca++).

  • Globulins: Classified as alpha, beta, and gamma globulins, function as antibodies, and transport lipids, iron, and copper.

  • Nutrients: Includes glucose, amino acids, fatty acids, vitamins.

  • Hormones: Includes waste products like urea and creatinine excreted through kidneys.

Laboratory Tests: Blood

Types of Blood Samples

1. Arterial Samples: Utilized for assessing arterial blood gases.

2. Venous Samples: Common tests include:

   • Complete Blood Count (CBC): Provides information on oxygen transport and the immune/inflammatory response.

     • Components of CBC include:

     • Red Blood Cell Count (RBC)

     • RBC indices:

       • Mean Corpuscular Volume (MCV)

       • Mean Corpuscular Hemoglobin (MCH)

       • MCH Concentration (MCHC)

     • Hemoglobin (Hb) level and Hematocrit (HCT): HCT measures RBC proportion in blood volume.

     • White Blood Cell Count (WBC)

     • Differential WBC count: Distribution of each WBC type in 100 WBCs.

Blood Clotting Mechanism

• After blood vessel injury:

  • Vascular Spasm: Injured vessel constricts to minimize blood loss temporarily.

  • Plug Formation: Platelets aggregate to form a temporary seal.

  • Coagulation: Conversion of fibrinogen to fibrin, forming a stable blood clot.

• Key Tests for Hemostasis:

  1. Platelets count

  2. Bleeding time

  3. Prothrombin time (PT/INR)

  4. Activated partial thromboplastin time (APTT)

  5. Fibrinogen level

Coagulation Pathways

• Extrinsic Pathway: Initiated by tissue damage outside of blood vessels leading to coagulation cascade.

• Intrinsic Pathway: Triggered by damage to blood vessel walls, wherein various clotting factors participate in a cascade.

Blood Chemistries

Electrolytes

• Cations: Sodium (Na+), Potassium (K+), Calcium (Ca++), Magnesium (Mg++)

• Anions: Chloride (Cl–), Bicarbonate (HCO3 –), Phosphate (HPO4 –)

Glucose & Hemoglobin A1C

• Glucose Levels: Regulated by glucagon and insulin.

• Hemoglobin A1C (Hgb A1C): Measures long-term blood sugar control; normal: 3% to 5% for non-diabetics.

  • Diabetics under control: 7% to 11%, poorly controlled: >11%.

Cardiac Markers

Key Cardiac Biomarkers

• Homocysteine: Elevated levels indicate increased cardiovascular disease risk.

• Brain Natriuretic Peptide (BNP): Found elevated in heart failure cases.

• C-Reactive Protein (CRP): Marker for inflammation response, useful for coronary artery disease screening.

• Arterial Blood Gas (ABG): Evaluates oxygenation and acid-base balance.

Lipid Profiles

Lipid Components

• Cholesterol: Steroid, cell membrane component and steroid precursor.

• Low-Density Lipoproteins (LDL): Responsible for cholesterol transport from liver to body; known as bad cholesterol.

• High-Density Lipoproteins (HDL): Transports excess cholesterol back to liver for breakdown; known as good cholesterol.

• Triglycerides: Made from fatty acids, proteins, and glucose.

• LDL Calculation: $LDL = ext{Total Cholesterol} - HDL - (0.20 imes ext{Total Triglycerides})$

Liver Function Tests

Key Components to Assess

• Albumin: Synthesized by liver; low levels indicate malnutrition.

• Prealbumin: Also synthesized by liver; more accurate malnutrition indicator.

• Bilirubin: Byproduct of hemolysis; elevated in jaundice cases.

• Alanine Aminotransferase (ALT): Enzyme indicating liver disease progression.

• Alkaline Phosphatase (ALP): Liver-specific enzyme.

• Aspartate Aminotransferase (AST): Released during liver, heart, or muscle cell death.

• Gamma-Glutamyltransferase (GGT): Elevated levels indicate liver disease.

Kidney Function Tests

Key Parameters

• Blood Urea Nitrogen (BUN): Byproduct of protein metabolism; elevated levels suggest renal impairment.

• Serum Creatinine: More sensitive than BUN for renal impairment.

• Normal BUN:Creatinine Ratio: 6:1 to 20:1.

Cardiac Markers in Myocardial Damage

• Used to detect myocardial damage; leak into bloodstream from injured heart cells.

• Creatine Kinase (CK): Enzyme in skeletal and cardiac muscle; CK-MB isoenzyme found in cardiac tissue.

• Myoglobin: Oxygen transport and storage protein released during myocardial damage.

• Troponin: Complex of proteins specific to cardiac muscle; detected within 4 hours post-MI.

Urinalysis

General Use

• Essential for diagnosing urinary tract infections and other non-renal disorders.

Stool Assessment

• Useful for identifying GI tract disorders. Includes tests for hidden blood, fat, and ova/parasites.

Stool Testing Notes

• Upper GI bleeding: Dark, tarry stools; lower GI bleeding: Bright red stools.

• Occult blood testing can detect as little as 5 mL/day of hidden blood.

• American Cancer Society recommends screening for colorectal cancer starting at age 50.

Radiography

Types and Purpose

• X-rays: Used for visualizing bones and soft tissues to identify abnormalities.

  • Non-contrast studies: No contrast material needed; include chest X-ray, bone imaging, mammography.

• Contrast Studies: Help visualize GI disorders via barium usage in upper and lower GI series.

Advanced Imaging Techniques

• CT Scans: Cross-sectional imaging using special scanners.

• MRI: Non-invasive, provides detailed images of soft tissues without radiation exposure.

• PET Scans: Visualize pathologies post radionuclide injection, useful in oncology.

• ECG: Records heart's electrical activity; assesses heart's state.

• Endoscopy: Used for direct visualization and intervention of internal organs.

Patient Guidelines for Diagnostic Tests

• Requires informed consent for many tests.

• Ensure patient understanding, preparation, and proper documentation by healthcare personnel.

Implementation and Evaluation**

• General responsibilities of nurses include monitoring vital signs, patient comfort, maintaining sterile fields, and ensuring specimens are labeled correctly.

Urine and Stool Collection Considerations

• Clean-catch, mid-stream collection for urine culture to minimize contamination.

• Proper instructions for stool samples; an adequate sample amount is crucial.

Summary of Best Practices

• Follow standard precautions during all specimen collections.

• Document any relevant information regarding the sample collections.

• Regular evaluations of processes and patient responses are critical to ensure quality care and effective testing procedures.