Clinical Laboratory Studies and Interpretation of Blood Gases
Chapter 7: Clinical Laboratory Studies
Learning Objectives
- After reading this chapter, you will be able to:
- Explain the three phases of laboratory testing.
- Describe the composition of blood.
- Explain the importance of specimen integrity and effects on laboratory test results.
- Define laboratory test sensitivity, specificity, and positive and negative predictive value.
- Discuss the meaning of the term reference range.
- Describe the clinical applications and general clinical significance of increases and decreases for each component of the complete blood count, the reticulocyte count, and erythrocyte sedimentation rate.
- Define leukocytosis, leukopenia, relative and absolute count, neutrophilia, neutropenia, lymphocytosis, lymphopenia, monocytosis, eosinophilia, basophilia, leukemia, anemia, and hemostasis.
- Describe the body's response to anemia and the potential effects on the body of uncompensated anemia.
- Differentiate primary, secondary, and relative polycythemia and describe the adverse effects of polycythemia on the body.
- Describe the clinical applications of the activated partial thromboplastin time, prothrombin time, and platelet count in assessing hemostasis.
- Explain the clinical application and significance of the quantitative d-dimer assay.
- Describe the clinical applications and general clinical significance of increases and decreases in electrolyte concentrations, glucose levels, blood urea nitrogen, and creatinine.
- Discuss the importance of renal and hepatic panel tests as related to the management of patients with respiratory disorders.
- Relate lipid panel measures to the risk for atherosclerosis and heart disease.
- Identify the current cardiac biomarkers used to help identify acute coronary syndrome and congestive heart failure.
- Describe the preanalytical phase of testing in clinical microbiology.
- Discuss the common methods of examination of microbiology specimens (e.g., Gram stain, acid-fast stains).
- Explain the purpose of a microbiology culture and antimicrobial sensitivity test.
- Describe the collection and transport protocols for pulmonary secretions.
- Discuss the importance of macroscopic and microscopic examination of sputum.
- List the microscopic criteria used to assess the quality of a sputum sample.
- Explain the significance of sputum eosinophilia.
- Describe the indications and method of performing a bronchoalveolar lavage.
- Describe the macroscopic, microscopic, and chemical significance of pleural fluid examination.
- Explain the purpose of histologic and cytologic examinations.
- List the malignant tumors responsible for producing most primary lung cancers.
- List the types of pulmonary samples that can be examined cytologically.
- Discuss the general concept of skin testing and two methods to screen for tuberculosis.
- Given a variety of patient presentations, identify the common laboratory tests helpful in assessing the problem.
Clinical Laboratory Overview
Clinical laboratory tests are used to:
- Diagnose, treat, monitor, and prevent disease.
Disciplines:
- Microbiology
- Hematology
- Immunology
- Transfusion medicine
- Clinical chemistry
- Molecular diagnostics
Phases of Laboratory Testing
Preanalytical Phase:
- Related to the selection of the appropriate assays for the patient’s condition, patient identification and preparation, and specimen selection, collection, and transport.
Analytical Phase:
- The actual testing performed by laboratory scientists.
Postanalytical Phase:
- Involves reporting and interpretation of results.
Blood Composition and Blood Cells
- Composition of Blood:
- Cells (45%):
- White blood cells (leukocytes)
- Red blood cells (erythrocytes)
- Platelets (thrombocytes)
- Plasma (55%):
- Water and soluble substances.
Laboratory Test Parameters
Usefulness:
- Predictive Value:
- Sensitivity:
- Frequency of positive test results in patients with disease.
- Specificity:
- Frequency of negative test results in patients without disease.
Definitions of Test Results:
- A True Positive (TP):
- A test is positive in a patient with the disease.
- A False Negative (FN):
- A test is negative in a patient with the disease.
- A True Negative (TN):
- A negative test in a disease-free patient.
- A False Positive (FP):
- A positive test occurs in a disease-free patient.
Hematology Tests
- Two Main Categories:
- General Hematology Tests:
- For evaluating normal and abnormal blood cells.
- Coagulation Studies:
- For evaluating blood clotting.
Complete Blood Count (CBC)
- Determines the number of circulating red and white blood cells.
- Determines the number and type of WBCs.
- Red blood cells are evaluated for size and the amount of hemoglobin present.
White Blood Cell (WBC) Count
- Total number of white blood cells in a known volume of blood.
- Distribution of white blood cell types:
- Neutrophils.
- Eosinophils.
- Lymphocytes.
- Basophils.
- Monocytes.
- In healthy individuals: Neutrophils and lymphocytes comprise the majority of the WBC count.
The Neutrophil
- Normally makes up 40% to 70% of the total white blood cell count.
- Produced in the bone marrow, where it matures and is called into action.
- Once a neutrophil leaves the bone marrow:
- Enters circulating blood.
- Marginates through the wall of the blood vessel and into surrounding tissues.
Other White Blood Cells
Eosinophils:
- Make up 0% to 6% of the WBCs.
- Play a role in allergic reactions.
Basophils:
- Make up 0% to 1% of the WBCs.
- Also play a role in allergic reactions.
Lymphocytes:
- Make up 20% to 45% of circulating WBCs.
- Useful in the fight against viral, fungal, and tuberculosis infections.
- Comprised of B cells and T cells (50-85% T cells; B cells and NK cells each approximately 5-20%).
Monocytes:
- Make up 2% to 10% of circulating WBCs.
- The largest type of WBC.
- In the lung, monocytes convert to macrophages and play a key role in clearing the lungs of inhaled dust through phagocytosis.
Nonmalignant White Blood Cell Abnormalities
- Leukocytosis:
- An abnormal increase in circulating WBCs.
- Neutrophilia:
- Increase of leukocytes due to neutrophils.
- Leukopenia:
- Abnormal decrease in circulating WBCs.
- Neutropenia:
- Drop in WBC caused by a decrease in neutrophils.
- Eosinophilia:
- Occurs with allergic reactions and parasitic infestations.
- Basophilia:
- Occurs with similar disorders that cause eosinophilia.
- Lymphocytosis:
- Occurs with viral infections, especially mononucleosis.
- Lymphocytopenia:
- A decrease in lymphocytes seen in immune deficiency states, including HIV infection.
- Monocytosis:
- Seen in chronic infections such as tuberculosis, and may occur in inflammatory conditions and autoimmune states.
Malignant White Blood Cell Abnormalities
- Leukemias:
- Involve the bone marrow and peripheral blood; characterized by accumulation of blasts, the most immature stage of a cell type.
- Lymphomas:
- Confined primarily to lymphoid tissue.
Red Blood Cells
- RBCs are produced in bone marrow and have a life span of 120 days.
- Main component of RBCs: Hemoglobin:
- Primary function is to carry oxygen to tissues.
- Important in maintaining acid-base balance (by acting as a buffer and transporting CO₂ from tissues to lungs).
Red Blood Cell Abnormalities
- Anemia:
- A low RBC count.
- Three RBC Indices:
- Mean Cell Volume (MCV)
- Mean Cell Hemoglobin (MCH)
- Mean Cell Hemoglobin Concentration (MCHC)
- When RBCs lack adequate hemoglobin, they are described as hypochromic.
- Polycythemia:
- High RBC count.
Reticulocyte Count
- Represents the final stage of erythrocyte development before the RBC is fully mature.
- Characteristics of immature RBCs:
- Lack a nucleus; slightly larger; have not yet assumed a biconcave shape.
Hemoglobin
- A protein that carries oxygen to tissues and is the major component of RBCs.
- It also plays a role in maintaining acid-base balance.
- Composition: Each hemoglobin molecule consists of four heme groups (each with an iron molecule capable of binding oxygen) and four globin chains.
Platelet Count
- Blood platelets (thrombocytes) are the smallest cells in peripheral blood.
- Abnormally low platelet count is termed thrombocytopenia.
- The lower the platelet count, the higher the risk of bleeding.
- Important for RTs to check platelet count before performing an arterial puncture.
Erythrocyte Sedimentation Rate (ESR)
- An ordered test to monitor inflammatory diseases.
- An increase in various plasma proteins reduces the negative charge on the surface of RBCs, leading to quicker aggregation and stacking of cells.
Coagulation Screening Tests
- Hemostasis:
- Three screening tests:
- Platelet count
- Activated Partial Thromboplastin Time (APTT)
- Prothrombin Time/International Normalized Ratio (PT/INR)
- Used to monitor anticoagulation therapy (e.g., patients on heparin and Coumadin).
- D-dimer:
- Produced as a result of the breakdown of fibrin clots that form in the vasculature.
Chemistry
- Measures biochemical compounds and external substances administered to patients.
- Basic Metabolic Panel Includes:
- Electrolytes
- Fasting glucose level
- Two renal function tests:
- Blood urea nitrogen (BUN)
- Creatinine
Electrolytes
- Sodium (Na+):
- Primary cation.
- Normal values: 135 to 145 mEq/L.
- Regulated by kidneys;
- Hypernatremia: Occurs from water loss.
- Hyponatremia: Caused by excessive water intake or sodium loss.
- Potassium (K+):
- Primary intracellular cation.
- Normal values: 3.5 to 5.0 mEq/L.
- Abnormal potassium levels can lead to cardiac dysfunction.
- Chloride (Cl-):
- Most common extracellular anion.
- Normal values: 98 to 107 mEq/L.
- Hypochloremia: Occurs with severe vomiting, chronic metabolic alkalosis.
- Hyperchloremia: Occurs with certain kidney diseases and prolonged diarrhea.
Total CO₂
- Plays a major role in acid-base balance.
- Elevation occurs with metabolic alkalosis;
- Decrease occurs with metabolic acidosis.
- COPD patients may have elevated total CO₂ due to chronic CO₂ retention.
Glucose
- Major carbohydrate in the blood.
- Common fasting blood sugar levels: 70 to 99 mg/dL.
- Hyperglycemia and Hypoglycemia.
Renal Panel
- BUN and Creatinine:
- Common tests to check on renal function; neither test is sensitive to early kidney disease; heart failure also elevates BUN.
- Glomerular Filtration Rate (GFR): Used to measure kidney function and assess kidney disease stage.
Hepatic Panel
- Measures proteins, including albumin and globulin, and liver-associated enzymes:
- Alkaline phosphatase (ALP)
- Alanine aminotransferase (ALT)
- Aspartate aminotransferase (AST)
- Bilirubin: A byproduct of the spleen's normal breakdown of hemoglobin.
Lipid Panel
- Measures lipids including total cholesterol:
- High-Density Lipoproteins (HDLs): < 40 mg/dL; referred to as good cholesterol because it removes excess cholesterol from circulation.
- Low-Density Lipoproteins (LDLs): < 130 mg/dL; referred to as bad cholesterol as it deposits cholesterol on blood vessel walls, potentially leading to atherosclerosis.
- Triglycerides: 30 to 149 mg/dL; the main storage form of fat in humans.
Cardiac Biomarkers
- Chemicals that appear in the blood due to ischemic myocardial damage or stress:
- Creatine kinase (CK)
- CK-MB
- Myoglobin
- Troponin
- B-type natriuretic peptide (BNP) used to help diagnose congestive heart failure.
Microbiology
- Clinical microbiology includes:
- Routine bacteriology
- Mycology
- Parasitology
- Virology
- Mycobacteriology
Specimen Sampling
- Preanalytical phase involves specimen selection, collection, and transport; microscopic examination; culture and sensitivity; and examination of pulmonary secretions including bronchoalveolar specimens (expectoration and induction).
Sputum Collection
- Expectorated Sputum: Preferred specimen for pneumonia diagnosis.
- Sputum Induction: Used hypertonic saline.
- Microscopic Examination: Assesses quality and provides information about microorganisms.
Bronchoalveolar Lavage (BAL)
- Performed during bronchoscopy, involves injecting large fluid volumes into the lungs, collecting after it mixes with cells.
- Used to diagnose interstitial lung disease and identify pneumonia causes.
Pleural Fluid Examination
Normal characteristics: clear and pale, volume of 3 to 20 mL, few WBCs, no RBCs.
Diseases or conditions diagnosed via:
- Exudate
- Transudate
Histology and Cytology
- Biopsy of lung tissue to evaluate benign vs malignant lesions.
- Cytology studies fluids and secretions for tumor cancer evaluations.
Skin Testing
- Tuberculin Skin Tests:
- PPD test
- Alternatives include interferon-gamma release assays (IGRA), such as QuantiFERON-TB Gold In-Tube test or T-SPOT TB Test.
Chapter 8: Interpretation of Blood Gases
Learning Objectives
- After reading this chapter, you will be able to:
- Identify the reference ranges for both arterial blood gas and oximetry parameters.
- Identify the indications for blood gas and oximetry analysis.
- Differentiate between invasive and noninvasive methods for measuring blood gas and oximetry parameters.
- Outline and explain the key procedural elements in obtaining arterial blood samples via puncture and indwelling arterial line.
- Apply knowledge of factors affecting hemoglobin saturation to interpret oximetry data.
- Apply common indices of oxygenation to assess the cause and severity of hypoxemia.
- Employ the Henderson-Hasselbalch equation to relate changes in PCO2 and HCO3- to pH.
- Describe common causes, compensatory mechanisms, and expected blood gas findings in simple respiratory and metabolic acid-base disorders.
- Describe common causes and expected blood gas findings in combined and mixed acid-base disorders.
- Identify common preanalytic, analytic, and postanalytic errors in blood gas analysis.
- Specify methods used to ensure valid measurement and use of blood gas data.
- Accurately interpret arterial blood gas and/or oximetry data.
Introduction
- Arterial blood O2 and CO2 levels reflect lung function, and analysis helps guide treatment.
- Mixed venous blood reflects tissue conditions; peripheral venous samples lack value.
- Pulse oximetry reduces the need for arterial blood gases (ABGs) but does not reflect CO2 levels or acid-base status.
Indications for Blood Gas and Oximetry Analysis
- Indicated if the patient’s symptoms, medical history, physical examination, or laboratory data suggest respiratory or acid-base abnormalities.
- Useful in evaluating treatment effectiveness, especially during significant therapy changes affecting oxygenation, ventilation, or acid-base balance.
Sampling and Measurement
- Invasive vs Noninvasive:
- Invasive approaches require blood sampling via needle puncture or indwelling catheter.
- Noninvasive methods measure blood gas parameters using sensors on the skin or evaluation of exhaled gases.
Invasive Blood Sampling
- Prior to arterial puncture, chart review to check blood clotting ability is essential.
- Low platelets or increased bleeding time will require longer post-puncture pressure.
- Normal compression time: 3 to 5 mins.
- Preferred Site: Radial artery due to accessibility, ease of stabilization, and collateral circulation from the ulnar artery.
- Alternative sites include brachial, femoral, or dorsalis pedis arteries.
Modified Allen's Test
- Assesses collateral circulation provided by the ulnar artery.
- Steps to Perform Test:
- Patient forms a fist.
- Compress both radial and ulnar arteries.
- Patient relaxes to reveal a blanched palm.
- Release pressure on ulnar artery and observe the time for the hand to return to color ("pink up").
- Adequate collateral flow occurs if pinking happens within 10 to 15 seconds.
Indwelling Catheter (A-line)
- Indications:
- Frequent blood sampling.
- Management of arterial BP; found in critically ill patients.
- Careful management needed to prevent hazards associated with setup and sampling.
Arterial Sample Handling
- Ensure air bubbles in the sample are removed to prevent inaccurate values from CO2 and O2 equilibrating with the bubbles.
- Samples not analyzed within 15 minutes need to be iced, but analysis must occur within 1 hour to limit cellular metabolic effects.
Noninvasive Measurements
- Pulse Oximetry: Measures SpO2.
- Transcutaneous Analysis: Noninvasive measurement of PtO2 and PtCO2.
- Capnography: Measures CO2 concentrations or partial pressures in expired gases; usually about 2 to 5 mm Hg lower than PaCO2.
Assessment of Oxygenation
- Parameters for Assessment:
- Arterial partial pressure of oxygen (PaO2)
- Hemoglobin content (Hb)
- Hemoglobin saturation (SaO2, SpO2)
- Arterial O2 content (CaO2)
Hypoxemia Assessment
PaO2 represents the pressure exerted by dissolved O2, reflecting the lungs' ability to transfer O2; levels gradually decrease with age.
Hypoxemia Categories:
- Mild
- Moderate
- Severe
Causes of Hypoxemia:
- V/Q mismatch
- Shunt
- Diffusion defects
- True hypoventilation
- Breathing reduced partial pressure of O2.
Hypoxia:
- Refers to inadequate tissue oxygenation.
Hemoglobin Saturation
- SaO2 reflects actual O2 bound to Hb compared with total capacity; >90% is generally considered adequate.
- CaO2: Total amount of oxygen carried in the blood (normal: 16 to 20 mL/dL); indicates dissolved (PaO2) and bound (to Hb) oxygen.
Quantifying Oxygenation and Hypoxemia
- PaO2:FiO2 (P:F) Ratio can be calculated to assess oxygenation.
- Oxygenation Index (OI): OI = (FiO2 x Paw)/PaO2, where Paw = Mean Airway Pressure.
- Alveolar Air Equation: PAO2 = F102(PB - H2O) - (Paco2 x 1.25).
Carboxyhemoglobin (HbCO)
- Amount of CO bound to Hb (normally 0% to 1%) measured by co-oximetry.
- CO competes with O2 for Hb binding sites having a 200 to 250 times' greater affinity than O2, decreasing O2 binding capacity.
Oxygen Dissociation Curve
- Changes in PaCO2, body temperature, and pH cause shifts in the O2-Hb curve, affecting oxygen transport and tissue oxygenation.
Clinical Assessment of Oxygen
- Goal: Provide adequate tissue oxygenation and evaluate:
- Lungs’ ability to oxygenate blood utilizing ABG analysis and pulse oximetry.
- Cardiovascular system role in blood distribution through physical assessment.
Interpretation of Blood Gas Measurements
- Arterial and mixed venous samples are useful to evaluate:
- Oxygen status by examining PaO2, SaO2, CO2, PvO2.
- Acid-base balance by examining pH, PaCO2, HCO3-, BE.
- Adequacy of ventilation by assessing PaCO2 levels.
Normal Arterial Blood Gas Values
- PaO2: 80 to 100 mm Hg
- SaO2: >95%
- CO2: 16 to 20 mL/dL blood
- pH: 7.35 to 7.45
- PaCO2: 35 to 45 mm Hg
- HCO3: 22 to 26 mEq/L
- BE: 0±2
- P(A-a)O₂: 10 to 15 mm Hg on room air
- Mixed venous oxygen (PvO2): 38 to 42 mm Hg.
Assessment of Acid-Base Balance
- pH: Reflects balance between blood acids and bases (normal: 7.35 to 7.45); pH
Partial Pressure of Carbon Dioxide (PaCO₂)
- An important respiratory component and reliable indicator of ventilation effectiveness.
- Hyperventilation: PaCO₂ <35 mm Hg causing respiratory alkalemia;
- Hypoventilation: PaCO₂ >45 mm Hg leading to respiratory acidemia.
Plasma Bicarbonate (HCO3-)
- Reflects the renal system's capacity to manage acids.
- HCO3 <22 mEq/L indicates metabolic acidemia;
- HCO3 >26 mEq/L indicates metabolic alkalemia.
Simple Acid-Base Disorders
Respiratory Acidosis:
- Caused by decreased alveolar ventilation, resulting in rising PaCO₂, leading to lowered pH.
- May result from pulmonary diseases, decreased drive to breathe, and diminished breathing abilities.
- Compensation occurs via renal retention of HCO3-.
For acute respiratory acidosis:
- Each 10 mm Hg increase in PaCO₂ results in a 1 mmol/L increase in plasma HCO3-.
Respiratory Alkalosis:
- Caused by excess alveolar ventilation, leading to decreased PaCO₂ and a rise in pH.
- Compensation occurs via renal loss of HCO3-.
Metabolic Acidosis:
- Defined by plasma HCO3- or BE falling below normal, often resulting from decreased buffer production or increased acid production.
- Compensation is achieved rapidly via hyperventilation.
- Lack of compensation suggests concurrent respiratory defects.
Simple Metabolic Alkalosis:
- Elevated plasma HCO3- or BE.
- Compensation through hypoventilation; comatose patients may exhibit significant response.
Mixed Acid-Base Disturbances
Occur when two simple acid-base disorders are present simultaneously.
May be difficult to identify when HCO3- and PaCO₂ move in opposite directions, complicating understanding of compensation levels.
Indicators of Mixed Disturbances:
- Significant changes in pH, PaCO₂, and HCO3-.
- Especially in critically ill patients, prognosis is often poor.
Ensuring Valid Measurements
- Avoiding errors is essential for accuracy in blood gas data collection.
- Preanalytic, analytic, and postanalytic errors must to be mitigated.
- Employ internal and external validity checks to ensure reliability.