What are Diagnostic Tests?
Diagnostic tests are medical procedures, laboratory analyses, and imaging studies used to identify diseases, monitor health conditions, assess organ function, and guide treatment decisions. These tests provide objective data that complement clinical examination and patient history to establish accurate diagnoses and monitor therapeutic responses.
Why Diagnostic Tests Matter:
- Enable early detection and prevention of diseases
- Provide objective measurements for clinical decision-making
- Monitor treatment effectiveness and disease progression
- Screen for asymptomatic conditions in at-risk populations
- Guide therapeutic interventions and medication dosing
- Establish baseline values for future comparison
Core Diagnostic Test Categories
Test Classification by Purpose
| Category | Purpose | Examples |
|---|---|---|
| Screening Tests | Detect disease in asymptomatic individuals | Mammography, colonoscopy, cholesterol panels |
| Diagnostic Tests | Confirm or rule out suspected conditions | ECG for chest pain, CT for abdominal pain |
| Monitoring Tests | Track disease progression or treatment response | HbA1c for diabetes, PSA for prostate cancer |
| Prognostic Tests | Predict disease outcomes | Genetic markers, tumor staging |
| Therapeutic Drug Monitoring | Optimize medication dosing | Digoxin levels, warfarin INR |
Test Classification by Method
| Method | Description | Clinical Applications |
|---|---|---|
| Laboratory Tests | Blood, urine, tissue analysis | Complete blood count, metabolic panels |
| Imaging Studies | Structural and functional visualization | X-rays, MRI, ultrasound, nuclear medicine |
| Physiological Tests | Organ function assessment | ECG, pulmonary function tests, stress tests |
| Pathological Tests | Tissue examination | Biopsies, cytology, histopathology |
| Genetic Tests | DNA/RNA analysis | Hereditary disease screening, pharmacogenomics |
Essential Laboratory Tests
Complete Blood Count (CBC)
| Parameter | Normal Range | Clinical Significance |
|---|---|---|
| Hemoglobin | M: 13.8-17.2 g/dL, F: 12.1-15.1 g/dL | Anemia, polycythemia |
| Hematocrit | M: 40.7-50.3%, F: 36.1-44.3% | Blood volume, oxygen-carrying capacity |
| White Blood Cells | 4,500-11,000/μL | Infection, immune disorders |
| Platelets | 150,000-450,000/μL | Bleeding disorders, clotting function |
| Red Blood Cells | M: 4.7-6.1 million/μL, F: 4.2-5.4 million/μL | Anemia, polycythemia |
CBC Differential
| Cell Type | Normal Range | Clinical Significance |
|---|---|---|
| Neutrophils | 50-70% | Bacterial infections, inflammation |
| Lymphocytes | 20-40% | Viral infections, immune disorders |
| Monocytes | 2-8% | Chronic infections, inflammatory conditions |
| Eosinophils | 1-4% | Allergies, parasitic infections |
| Basophils | 0.5-1% | Allergic reactions, myeloproliferative disorders |
Basic Metabolic Panel (BMP)
| Parameter | Normal Range | Clinical Significance |
|---|---|---|
| Glucose | 70-99 mg/dL (fasting) | Diabetes, hypoglycemia |
| Sodium | 136-145 mEq/L | Fluid balance, kidney function |
| Potassium | 3.5-5.0 mEq/L | Cardiac function, kidney disease |
| Chloride | 98-107 mEq/L | Acid-base balance |
| CO2 | 22-29 mEq/L | Acid-base status |
| BUN | 7-20 mg/dL | Kidney function |
| Creatinine | M: 0.74-1.35 mg/dL, F: 0.59-1.04 mg/dL | Kidney function |
Comprehensive Metabolic Panel (CMP)
Includes BMP plus:
| Parameter | Normal Range | Clinical Significance |
|---|---|---|
| Total Protein | 6.3-8.2 g/dL | Nutritional status, liver function |
| Albumin | 3.5-5.0 g/dL | Liver function, nutritional status |
| Total Bilirubin | 0.3-1.2 mg/dL | Liver function, hemolysis |
| AST (SGOT) | 10-40 U/L | Liver damage, muscle injury |
| ALT (SGPT) | 7-56 U/L | Liver damage (more specific) |
| Alkaline Phosphatase | 44-147 U/L | Liver, bone disorders |
Lipid Panel
| Parameter | Optimal Range | Clinical Significance |
|---|---|---|
| Total Cholesterol | <200 mg/dL | Cardiovascular risk |
| LDL Cholesterol | <100 mg/dL | “Bad” cholesterol, atherosclerosis |
| HDL Cholesterol | M: >40 mg/dL, F: >50 mg/dL | “Good” cholesterol, cardioprotective |
| Triglycerides | <150 mg/dL | Cardiovascular risk, pancreatitis |
| Non-HDL Cholesterol | <130 mg/dL | Atherosclerotic risk |
Thyroid Function Tests
| Parameter | Normal Range | Clinical Significance |
|---|---|---|
| TSH | 0.27-4.2 mIU/L | Primary screening for thyroid disorders |
| Free T4 | 0.93-1.7 ng/dL | Thyroid hormone production |
| Free T3 | 2.0-4.4 pg/mL | Active thyroid hormone |
| Total T4 | 4.5-12.0 μg/dL | Total thyroxine levels |
| Total T3 | 80-200 ng/dL | Total triiodothyronine |
Cardiac Diagnostic Tests
Cardiac Enzymes and Markers
| Marker | Normal Range | Peak Time | Clinical Use |
|---|---|---|---|
| Troponin I | <0.04 ng/mL | 12-24 hours | MI diagnosis (most specific) |
| Troponin T | <0.01 ng/mL | 12-24 hours | MI diagnosis |
| CK-MB | 0-6.3 ng/mL | 12-24 hours | MI diagnosis (less specific) |
| Total CK | M: 55-170 U/L, F: 30-135 U/L | 12-24 hours | Muscle damage |
| BNP | <100 pg/mL | N/A | Heart failure |
| NT-proBNP | <125 pg/mL | N/A | Heart failure (more sensitive) |
Electrocardiogram (ECG) Interpretation
| Parameter | Normal Values | Abnormal Findings |
|---|---|---|
| Heart Rate | 60-100 bpm | Bradycardia (<60), tachycardia (>100) |
| PR Interval | 0.12-0.20 seconds | AV blocks, pre-excitation |
| QRS Duration | <0.12 seconds | Bundle branch blocks, ventricular rhythms |
| QT Interval | <440 ms (corrected) | Long QT syndrome, drug effects |
| Axis | -30° to +90° | Left/right axis deviation |
Infectious Disease Tests
Common Bacterial Culture Sites
| Site | Normal Flora | Common Pathogens |
|---|---|---|
| Blood | Sterile | Staphylococcus, Streptococcus, E. coli |
| Urine | <10⁵ CFU/mL | E. coli, Enterococcus, Pseudomonas |
| Throat | Normal flora present | Group A Strep, Group C/G Strep |
| Sputum | Normal flora present | S. pneumoniae, H. influenzae |
| Wound | Variable | Staphylococcus, Streptococcus, Pseudomonas |
Serological Tests
| Test | Purpose | Interpretation |
|---|---|---|
| Hepatitis Panel | Viral hepatitis screening | HBsAg, anti-HBc, anti-HCV |
| HIV Testing | HIV infection | ELISA + Western blot/PCR |
| Syphilis (RPR/VDRL) | Syphilis screening | Titer indicates activity |
| Monospot | Infectious mononucleosis | EBV infection |
| Lyme Titer | Lyme disease | ELISA + Western blot |
Urinalysis
Physical Examination
| Parameter | Normal | Abnormal Findings |
|---|---|---|
| Color | Yellow to amber | Red (blood), dark (concentrated) |
| Clarity | Clear | Cloudy (infection, crystals) |
| Specific Gravity | 1.003-1.030 | High (dehydration), low (diabetes insipidus) |
Chemical Analysis
| Parameter | Normal | Clinical Significance |
|---|---|---|
| Protein | Negative to trace | Kidney disease, hypertension |
| Glucose | Negative | Diabetes mellitus |
| Ketones | Negative | Diabetic ketoacidosis, starvation |
| Blood | Negative | UTI, kidney stones, trauma |
| Leukocyte Esterase | Negative | Urinary tract infection |
| Nitrites | Negative | Bacterial infection (gram-negative) |
Microscopic Examination
| Finding | Normal | Abnormal Significance |
|---|---|---|
| RBCs | 0-2/hpf | Hematuria, kidney disease |
| WBCs | 0-5/hpf | Urinary tract infection |
| Bacteria | Few | UTI if >10⁵ CFU/mL |
| Casts | Occasional hyaline | RBC casts (glomerulonephritis) |
| Crystals | Variable | Kidney stones, metabolic disorders |
Imaging Studies
Chest X-Ray Interpretation
| Finding | Normal | Abnormal Possibilities |
|---|---|---|
| Heart Size | <50% thoracic width | Cardiomegaly, heart failure |
| Lung Fields | Clear, equal | Pneumonia, pneumothorax, effusion |
| Mediastinum | Central, normal width | Masses, lymphadenopathy |
| Diaphragm | Clear, dome-shaped | Elevation, eventration |
| Costophrenic Angles | Sharp | Pleural effusion |
CT Scan Considerations
| Body System | Common Indications | Contrast Requirements |
|---|---|---|
| Head | Stroke, trauma, headache | Usually without contrast |
| Chest | Pulmonary embolism, masses | With IV contrast |
| Abdomen/Pelvis | Abdominal pain, infection | Oral and IV contrast |
| Cardiac | Coronary artery disease | IV contrast with gating |
MRI Applications
| System | Best For | Limitations |
|---|---|---|
| Brain/Spine | Soft tissue detail, tumors | Claustrophobia, metal implants |
| Musculoskeletal | Ligaments, cartilage | Time-consuming, expensive |
| Cardiac | Function assessment | Arrhythmias interfere |
| Abdominal | Liver lesions, bile ducts | Motion artifacts |
Tumor Markers
Common Cancer Markers
| Marker | Associated Cancers | Normal Range | Clinical Use |
|---|---|---|---|
| PSA | Prostate | <4.0 ng/mL | Screening, monitoring |
| CEA | Colorectal, lung, breast | <3.0 ng/mL | Monitoring, not screening |
| CA 19-9 | Pancreatic, biliary | <37 U/mL | Monitoring treatment |
| CA 125 | Ovarian | <35 U/mL | Monitoring, high false positives |
| AFP | Liver, testicular | <10 ng/mL | Diagnosis, monitoring |
| β-hCG | Testicular, gestational | <2 mIU/mL (non-pregnant) | Diagnosis, monitoring |
Coagulation Studies
Clotting Tests
| Test | Normal Range | Clinical Significance |
|---|---|---|
| PT (Prothrombin Time) | 11-13 seconds | Warfarin monitoring, liver function |
| INR | 0.8-1.1 | Standardized PT for warfarin |
| aPTT | 25-35 seconds | Heparin monitoring, clotting disorders |
| Platelets | 150,000-450,000/μL | Bleeding risk assessment |
| Fibrinogen | 200-400 mg/dL | Clotting function, DIC |
| D-dimer | <500 ng/mL | Pulmonary embolism, DVT |
Endocrine Function Tests
Diabetes Monitoring
| Test | Target Range | Clinical Use |
|---|---|---|
| Fasting Glucose | 70-99 mg/dL | Diabetes screening |
| Random Glucose | <140 mg/dL | Diabetes screening |
| HbA1c | <7% (diabetics) | Long-term glucose control |
| Oral Glucose Tolerance | <140 mg/dL (2-hour) | Diabetes diagnosis |
| C-peptide | 0.5-2.0 ng/mL | Insulin production assessment |
Adrenal Function
| Test | Normal Range | Clinical Significance |
|---|---|---|
| Cortisol (morning) | 6-23 μg/dL | Adrenal function |
| ACTH | 9-52 pg/mL | Pituitary-adrenal axis |
| Aldosterone | 4-31 ng/dL | Mineralocorticoid function |
| Catecholamines | Variable | Pheochromocytoma |
Diagnostic Test Interpretation Principles
Sensitivity and Specificity
| Concept | Definition | Clinical Implication |
|---|---|---|
| Sensitivity | True positive rate | High sensitivity = good screening test |
| Specificity | True negative rate | High specificity = good confirmatory test |
| Positive Predictive Value | Probability of disease when test positive | Depends on disease prevalence |
| Negative Predictive Value | Probability of no disease when test negative | Depends on disease prevalence |
Reference Ranges and Critical Values
| Concept | Description | Clinical Action |
|---|---|---|
| Reference Range | 95% of healthy population | Values outside may be normal variants |
| Critical Values | Life-threatening results | Immediate physician notification required |
| Panic Values | Extremely abnormal results | Urgent clinical intervention needed |
| Delta Checks | Significant changes from previous | Review for accuracy and clinical correlation |
Common Diagnostic Challenges
Frequent Issues and Solutions
| Problem | Cause | Solution |
|---|---|---|
| False Positives | Low specificity, cross-reactivity | Confirm with additional tests |
| False Negatives | Low sensitivity, timing issues | Repeat testing, use more sensitive methods |
| Interfering Substances | Medications, diet, hemolysis | Pre-test preparation, proper collection |
| Reference Range Variations | Age, sex, ethnicity differences | Use appropriate reference ranges |
| Pre-analytical Errors | Collection, transport, storage issues | Standardize procedures, training |
Patient Preparation Requirements
| Test Type | Preparation Needed | Duration |
|---|---|---|
| Fasting Labs | No food or drink (except water) | 8-12 hours |
| Glucose Tolerance | Fasting + specific glucose load | 8-12 hours fasting |
| Lipid Panel | Fasting preferred | 9-12 hours |
| Thyroid Tests | Avoid biotin supplements | 2-3 days |
| Drug Levels | Timing relative to dosing | Test-specific |
Quality Control and Safety
Laboratory Quality Measures
| Measure | Purpose | Frequency |
|---|---|---|
| Internal QC | Monitor precision and accuracy | With each batch |
| External QC | Proficiency testing | Monthly/quarterly |
| Calibration | Ensure instrument accuracy | Daily/per manufacturer |
| Maintenance | Prevent instrument failure | Scheduled intervals |
Safety Considerations
- Radiation Exposure: ALARA principle (As Low As Reasonably Achievable)
- Contrast Reactions: Screen for allergies, kidney function
- Infection Control: Universal precautions for all specimens
- Patient Identification: Two identifiers minimum
- Result Communication: Critical values protocols
Emerging Diagnostic Technologies
Next-Generation Diagnostics
| Technology | Applications | Advantages |
|---|---|---|
| Point-of-Care Testing | Rapid bedside results | Fast turnaround, convenience |
| Molecular Diagnostics | Genetic testing, infectious diseases | High specificity, personalized medicine |
| Artificial Intelligence | Image interpretation, pattern recognition | Improved accuracy, efficiency |
| Liquid Biopsies | Cancer detection via blood | Non-invasive, early detection |
| Wearable Devices | Continuous monitoring | Real-time data, patient engagement |
Quick Reference Guides
Critical Value Ranges (Immediate Notification Required)
| Parameter | Critical Low | Critical High |
|---|---|---|
| Glucose | <50 mg/dL | >400 mg/dL |
| Potassium | <2.5 mEq/L | >6.0 mEq/L |
| Sodium | <120 mEq/L | >160 mEq/L |
| Hemoglobin | <7.0 g/dL | >20 g/dL |
| Platelets | <50,000/μL | >1,000,000/μL |
| Creatinine | N/A | >5.0 mg/dL |
Common Drug Therapeutic Ranges
| Drug | Therapeutic Range | Toxic Level |
|---|---|---|
| Digoxin | 1.0-2.0 ng/mL | >2.0 ng/mL |
| Lithium | 0.6-1.2 mEq/L | >1.5 mEq/L |
| Phenytoin | 10-20 μg/mL | >20 μg/mL |
| Theophylline | 10-20 μg/mL | >20 μg/mL |
| Vancomycin | 15-20 μg/mL (trough) | >20 μg/mL |
Specimen Collection Priorities
| Tube Color | Additive | Common Tests |
|---|---|---|
| Red | None (serum) | Chemistry, serology |
| Lavender | EDTA | CBC, blood bank |
| Blue | Citrate | Coagulation studies |
| Green | Heparin | Chemistry (stat) |
| Gray | Fluoride | Glucose, lactate |
| Yellow | SPS | Blood cultures |
Resources for Further Learning
Professional Organizations
- American Association for Clinical Chemistry (AACC)
- College of American Pathologists (CAP)
- Clinical Laboratory Management Association (CLMA)
- American Society for Clinical Laboratory Science (ASCLS)
Clinical Guidelines
- Centers for Disease Control and Prevention (CDC) Laboratory Guidelines
- Clinical and Laboratory Standards Institute (CLSI) Standards
- World Health Organization (WHO) Laboratory Guidelines
- American Medical Association (AMA) Laboratory Testing Guidelines
Educational Resources
- Lab Tests Online: Patient-friendly test explanations
- Medscape Laboratory Medicine: Clinical interpretation guides
- Up-to-Date: Evidence-based clinical decision support
- NEJM Knowledge+: Medical education platform
Reference Materials
- Tietz Textbook of Clinical Chemistry: Comprehensive laboratory medicine
- Henry’s Clinical Diagnosis and Management: Clinical laboratory methods
- Wallach’s Interpretation of Diagnostic Tests: Clinical correlation guide
- ARUP Laboratory Test Directory: Comprehensive test catalog
Continuing Education
- American Board of Clinical Chemistry: Certification programs
- Association of Public Health Laboratories: Training courses
- College of American Pathologists: Educational resources
- Local hospital laboratory continuing education programs