642 Diagnostic Testing

Diagnostic Studies

  • Role of History and Physical Examination

    • Guides the selection of diagnostic studies.

    • Establishes severity of a condition.

    • Monitors particular therapy.

    • Rules out conditions included in differential diagnoses.

Selecting Diagnostic Studies

  • Selection Criteria

    • Choose studies with the highest degree of sensitivity and specificity for the target condition.

    • Considerations include cost-effectiveness, safety, and invasiveness.

Definitions & Key Terms

  • Validity: Whether the test accurately identifies disease presence.

  • Sensitivity: The probability that a person with a disease has a positive test (true positive).

  • Specificity: The probability that a person without a disease has a negative test (true negative).

  • SnNOUT: Sensitivity helps rule out disease when the test is negative.

  • SpPIN: Specificity helps rule in disease when the test is positive.

  • Positive Predictive Value (PPV): Probability that a person with a positive test actually has the disease.

  • Negative Predictive Value (NPV): Probability that a person with a negative test does not have the disease.

2x2 Table for Diagnostic Test Values

  • Structure:

    • Gold Standard: Disease Present

    • Gold Standard: Disease Absent

    • Positive test:

      • a (true positive)

      • b (false positive)

      • Total positive results (a + b)

    • Negative test:

      • c (false negative)

      • d (true negative)

      • Total negative results (c + d)

    • Totals:

    • Total with disease: (a + c)

    • Total without disease: (b + d)

    • Total in study: (a + b + c + d)

B-type Natriuretic Peptide (BNP) for Diagnosing Heart Failure

  • Test Results:

    • Heart Failure Present:

    • BNP ≥ 100:

      • 650 true positives

      • 220 false positives

      • 870 total positive results

    • Heart Failure Absent:

    • BNP < 100:

      • 72 false negatives

      • 596 true negatives

      • 668 total negative results

  • Total Counts:

    • 722 total with disease

    • 816 total without disease

    • 1538 total in study

  • Calculations for Sensitivity, Specificity, PPV, NPV:

    • Sensitivity = aa+c=650722=0.9\frac{a}{a + c} = \frac{650}{722} = 0.9 (90%)

    • Specificity = db+d=596816=0.73\frac{d}{b + d} = \frac{596}{816} = 0.73 (73%)

    • PPV = aa+b=650870=0.75\frac{a}{a + b} = \frac{650}{870} = 0.75 (75%)

    • NPV = dc+d=596668=0.89\frac{d}{c + d} = \frac{596}{668} = 0.89 (89%)

  • Prevalence of Heart Failure in Study:

    • McCullough et al., 2002: 47%

The Diagnostic Process

  • Steps:

    • Data collection (History & Physical Examination)

    • Develop differential diagnoses: list of common causes requiring urgent action.

    • Perform diagnostic tests to confirm or rule out differential diagnoses.

    • If uncertainty remains, consider:

    • Further history & physical examinations

    • Additional diagnostic tests

    • Referral to specialists

    • Schedule serial assessments to monitor decision-making.

    • Consider patient expectations in the process.

Interpreting Blood Tests

Interpreting Blood Test Results

  • Key Questions:

    1. How far out of range is the result?

    2. Do we have a clinical decision limit or guideline?

    3. Does the result make sense in the clinical context?

    4. Do familial group norms support the results?

Complete Blood Count (CBC)

Key Components

  • Red Blood Cells (RBC):

    • Total number of red blood cells in blood.

  • Hemoglobin (Hgb):

    • Oxygen-carrying protein; reflects the number of RBCs.

  • Hematocrit (Hct):

    • Percentage of red blood cells in whole blood.

  • Mean Cell Volume (MCV):

    • Average size of red blood cells; aids in identifying types of anemia.

  • White Blood Cells (WBC):

    • Total number of white blood cells in blood.

    • Neutrophils: Responds to tissue damage.

    • Lymphocytes: Produces antibodies.

    • Monocytes: Infiltrates tissue in systemic bacterial infections.

    • Basophils and Eosinophils: Important in allergic responses and hypersensitivity.

  • Platelets (PLT):

    • Number of platelets that cause blood to clot; also a marker of bone marrow function.

Family Groups in Blood Counts

  • Complete Blood Count (CBC):

    • Hemoglobin, White Blood Cells, Platelets; gives information on bone marrow function.

  • Red Blood Cells Analysis:

    • Hemoglobin and Hematocrit; used for evaluating anemia.

  • White Blood Cells Types:

    • Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils; provides insights on inflammation and infection.

Case Study Values

  • Values Observed:

    • WBC, RBC, HGB, HCT, MCV, PLT

  • Normal Count Example:

    • WBC: 9.4 K/UL (Normal Range: 4.0-10.0 K/UL)

    • RBC: 4.81 M/UL (Normal Range: 3.6-5.5 M/UL)

    • HGB: 13.7 GM/DL (Normal Range: 12.0-16.0 GM/DL)

    • HCT: 41.1% (Normal Range: 34.0-51.0%)

    • MCV: 85.4 FL (Normal Range: 85-95 FL)

    • PLT: 220 K/UL (Normal Range: 150-400 K/UL)

  • Low/High Count Example:

    • WBC: 19.5 (H)

    • RBC: 3.49 (L)

    • HGB: 10.4 (L)

    • HCT: 31.2 (L)

    • MCV: 82 (L)

    • PLT: 98 (L)

Documenting the CBC

  • Clinician Shorthand:

    • Example from CBC:

    • WBC: 19.5, HGB: 10.4, HCT: 31.2, PLT: 98.

Basic Metabolic Panel (BMP)

Components

  • Glucose:

    • Measure of sugar in blood.

  • Sodium (Na):

    • Extracellular electrolyte controlling fluid balance and blood pressure.

  • Potassium (K):

    • Intracellular electrolyte controlling cellular functions via electric potential.

  • Chloride:

    • Extracellular electrolyte, also influencing fluid balance and blood pressure.

  • Carbon Dioxide:

    • Represents bicarbonate level, which acts as a chemical buffer controlling pH.

  • Blood Urea Nitrogen (BUN):

    • Marker of acute renal dysfunction (e.g., acute distress, dehydration).

  • Creatinine:

    • Indicator of chronic renal function.

  • Calcium:

    • Essential for muscle, nerve, and heart function, and blood clotting.

  • Estimated Glomerular Filtration Rate (eGFR):

    • A measure of kidney function used to assist in the diagnosis and staging of chronic kidney disease.

Documenting the BMP

  • Clinician Shorthand Example:

    • Last BMP recorded values:

    • Na: 142, K: 3.9, Cl: 107, CO2: 27, BUN: 10, Cr: 0.8, Glu: 100.

Comprehensive Metabolic Panel

Components

  • Total Protein:

    • Measures albumin and other proteins in serum.

  • Albumin:

    • Protein produced by liver; helps maintain osmotic pressure; decreased in liver damage.

  • Bilirubin:

    • Indicates liver conditions, increasing with jaundice due to hepatic blockages.

  • Liver Enzymes:

    • Alanine aminotransferase (ALT), Aspartate aminotransferase (AST), Gamma-glutamyl transferase (GGT), Alkaline phosphatase (ALP); indicators of liver function or damage.

Documenting the CMP

  • Included Parameters:

    • Liver Function tests: ALT, Albumin, Bilirubin, ALP, AST.

    • Renal Function tests: Sodium, Potassium, BUN, Creatinine, eGFR, GGT.

Case Study Values

  • Example 1:

    • Glucose: 112 (H), BUN: 39 (H), Creatinine: 1.6 (H), Calcium: 8.9, Sodium: 132 (L), Potassium: 4.0, Chloride: 93 (L), Carbon Dioxide: 23, Albumin: 3.1 (L), Total Protein: 5.8 (L), Alkaline Phosphatase: 200, AST: 35, Bilirubin: 1.9 (H)

    • Patient: 68 yo male with acute on chronic systolic heart failure.

Coagulation Tests

  • Key Measurements:

    • International Normalized Ratio (INR)

    • Prothrombin Time (PT)

    • Activated Partial Thromboplastin Time (aPTT)

    • D-dimer

Disease Markers

  • Diabetes:

    • Fasting glucose level, Oral glucose tolerance test, Hemoglobin A1c.

  • Cholesterol:

    • Total cholesterol, LDL, HDL, Triglycerides.

  • Thyroid function test (TFT):

    • Thyroid stimulating hormone (TSH), T3, T4.

Urinalysis: Normal Findings

  • Parameters:

    • Appearance: Clear

    • Color: Amber yellow

    • Odor: Aromatic

    • pH: 4.6 – 8

    • Protein: 0 – 8 mg/dL

    • Specific Gravity: 1.005 – 1.030

    • Leukocyte Esterase: Negative

    • Nitrites: None

    • Ketones: None

    • Bilirubin: None

    • Urobilinogen: 0.01 – 1 Ehrlich unit/mL

    • Crystals: None

    • Casts: None

    • Glucose: Negative

    • White Blood Cells: 0 – 4/low-power field

    • WBC Casts: None

    • Red Blood Cells (RBCs): ≤ 2

    • RBC Casts: None.

Important Note

  • Ethical Practice:

    • No lab test should ever be ordered unless it is medically necessary.

Chest X-Ray

Initial Interpretation

  • Image Date: 7/30/99, SE 78.

Anatomical Landmarks

  • Key Structures:

    • Scapula, Clavicle, Manubrium Sternum, Rib cage, Azygoesophageal line, Descending Aorta, Costophrenic angles, the left and right hemidiaphragms, Left ventricle, Posterior wall bronch, etc.

Interpretation Guidelines

  • Inspiration Check: Diaphragm at level of 9th-10th rib

  • Density Visualization: Ensure visualization of thoracic spine through heart shadow, intervertebral discs in cervical spine.

  • Rotation Check: Clavicles, ribs, and spinal processes should be symmetrical and equal heights bilaterally.

Penetration

  • Categories:

    • Over Penetration

    • Under Penetration

Reading Guidelines: ABCDEs

  • Adequacy

  • Airway

  • Breathing

  • Cardiac

  • Diaphragm Edges

  • Edges of Skeletal/Soft Tissue.

Airway and Cardiac Evaluation

  • Airway: Trachea, Carina, Right & Left Main Bronchi.

  • Cardiac: Identify cardiac silhouette, Right/Left Hilum, Aortic Knob, and the Costocardiac Angle.

Diaphragm Observations

  • Right hemidiaphragm sits higher than left hemidiaphragm.

Edges and Structures

  • Edges: Should be crisp and clearly visualized.

  • Skeleton and Soft Tissue: Evaluate external structures for fractures, neoplasms, free air, foreign bodies.

Silhouette Sign

  • Definition: Loss of silhouette or lung/soft tissue interface signifies mass or fluid presence in normally air-filled lung.

Pneumonia Evaluation

  • Right Middle Lobe Pneumonia:

    • Airspace disease with consolidation. X-ray findings include airspace opacity and lobar consolidation.

Heart Failure (HF)

  • Definition: HF occurs when the heart fails to maintain adequate forward flow.

  • Potential progression to pulmonary venous hypertension and pulmonary edema with leakage of fluid into interstitium, alveoli, and pleural spaces.

Emphysema Overview

  • Condition characterized by loss of lung's elastic recoil, destruction of pulmonary capillary bed, and alveolar septa.

  • Hallmarks: Decreased airflow (decreased FEV1) and diffusing capacity (decreased DLCO2).

  • Common radiographic findings: diffuse hyperinflation, diaphragm flattening, increased retrosternal space, bullae, and PA/RV enlargement (cor pulmonale).

Other Conditions

  • Pleural Effusion: Fluid build-up between pleurae.

  • Lung Cancer: Example, Right lower lobe squamous cell carcinoma.