Lactate Dehydrogenase (LDH Pleural Fluid)

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Measures LDH enzyme.

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Lactate Dehydrogenase (LDH Pleural Fluid) - Comprehensive Medical Test Guide

Section 1: Why is it done?
- Test Purpose: LDH pleural fluid measurement quantifies lactate dehydrogenase enzyme levels in fluid collected from the pleural space (between the lungs and chest wall). This test helps differentiate between transudate and exudate pleural effusions and aids in diagnosing the underlying cause of fluid accumulation.
- Primary Indications: Evaluation of pleural effusion to determine if fluid is an exudate (indicating disease) or transudate (indicating systemic condition); Assessment for malignancy, infection, or inflammation; Diagnosis of pneumonia, tuberculosis, pulmonary embolism, or rheumatologic disorders; Monitoring of pleural disease progression
- Timing/Circumstances: Performed during thoracentesis (needle aspiration of pleural fluid); Usually ordered when imaging shows pleural effusion of unknown origin; Can be repeated if clinical status changes or diagnosis remains unclear; Part of routine pleural fluid analysis panel
Section 2: Normal Range
- Reference Values: Normal pleural fluid LDH: Less than 200 IU/L (or <2/3 of serum LDH upper limit); Typical range: 50-150 IU/L in non-inflamed pleural fluid; Values vary slightly by laboratory and methodology
- Units of Measurement: International Units per Liter (IU/L); Some laboratories report in μmol/(min•mL) or other enzymatic units depending on assay method
- Interpretation of Results: Normal (Low) LDH (<200 IU/L): Suggests transudate, indicates systemic cause such as heart failure, liver cirrhosis, or kidney disease; Fluid is ultrafiltrate of plasma with intact membranes; Elevated LDH (>200 IU/L, especially >400 IU/L): Indicates exudate suggesting local pleural disease; LDH pleural/serum ratio >0.6 is considered exudative; Suggests infection, malignancy, inflammation, or tissue damage in pleural space
- Normal vs Abnormal: Normal (transudative) pleural fluid has low LDH reflecting minimal cellular activity and intact pleural membranes; Abnormal (exudative) pleural fluid has elevated LDH due to cellular inflammation, tissue breakdown, and increased membrane permeability; Used with protein levels in Light's criteria to classify effusions
Section 3: Interpretation
- Detailed Result Interpretation: LDH <200 IU/L (Low): Indicates transudate; Associated with systemic conditions like congestive heart failure, cirrhosis, nephrotic syndrome, or pulmonary embolism; Suggests pleural membranes are intact with passive fluid accumulation; LDH 200-400 IU/L (Borderline Elevated): May indicate early exudative process or borderline exudate/transudate; Requires correlation with protein levels and pleural/serum LDH ratio; Clinical context becomes important for interpretation; LDH >400 IU/L (Significantly Elevated): Strongly suggests exudate with active pleural disease; Indicates infection, malignancy, severe inflammation, or significant tissue damage; Higher values often seen in empyema, tuberculosis, or malignant effusion
- Clinical Significance of Patterns: Pleural LDH/Serum LDH Ratio >0.6: Highly specific for exudate; Helps distinguish true exudates from pseudo-transudates; More reliable than absolute LDH values; Progressive LDH Elevation: Suggests worsening pleural inflammation or infection; May indicate treatment failure or disease progression; Warrants reevaluation of diagnosis and therapy; Combined with Low Glucose: Suggests bacterial infection or rheumatoid arthritis; Combined with Low pH: Indicates empyema or complicated parapneumonic effusion; Combined with Positive Culture: Confirms infectious etiology
- Factors Affecting Results: Sample quality: Hemolysis or contamination increases LDH; Specimen must be collected in appropriate tube and processed promptly; Time delay may falsely increase LDH; Serum LDH levels: Must be measured on same day for ratio calculation; Serum LDH elevation affects interpretation; Clinical conditions: Recent thoracentesis, pleurodesis, or chest trauma can elevate LDH; Anticoagulation therapy generally does not affect LDH levels; Disease activity: Chronic stable conditions may have lower LDH than acute disease; Recent chemotherapy or radiation may elevate levels
Section 4: Associated Organs
- Primary Organ Systems: Pulmonary/Respiratory System: Lungs and pleura are directly involved; Pleural membranes produce and reabsorb fluid; LDH reflects pleural inflammation; Cardiovascular System: Congestive heart failure is common cause of transudate; Right heart failure increases venous pressure; Systemic venous congestion contributes to effusion; Hepatic System: Cirrhosis causes decreased plasma proteins and transudative effusion; Portal hypertension increases venous pressure; Renal System: Nephrotic syndrome causes protein loss and transudate formation; Kidney disease affects albumin levels and oncotic pressure
- Medical Conditions Associated with Abnormal Results: Infections (Exudative, High LDH): Pneumonia with parapneumonic effusion; Tuberculosis; Empyema; Fungal infections; Viral infections; Malignancy (Exudative, Very High LDH): Primary lung cancer; Metastatic disease; Lymphoma; Mesothelioma; Inflammatory/Autoimmune (Exudative, Moderate-High LDH): Systemic lupus erythematosus; Rheumatoid arthritis; Pulmonary embolism; Post-cardiac surgery syndrome; Systemic Conditions (Transudative, Low LDH): Congestive heart failure; Cirrhosis; Nephrotic syndrome; Hypoalbuminemia; Other Conditions: Pancreatitis; Esophageal rupture; Uremia; Radiation therapy effects
- Diseases This Test Helps Diagnose: Exudative pleural effusion of unknown origin; Parapneumonic effusion vs uncomplicated; Tuberculosis pleuritis; Malignant pleural effusion; Empyema; Rheumatological pleural disease; Pulmonary embolism with infarction; Post-cardiac surgery effusion; Pleuritis from various causes
- Potential Complications of Abnormal Results: If indicating severe infection (empyema): Risk of loculation, sepsis, fibrothorax, respiratory compromise; Requires urgent drainage and antibiotics; Mortality increases with delayed treatment; If indicating malignancy: Represents advanced/metastatic disease; Poor prognosis marker; Functional decline and shortened survival; If indicating pulmonary embolism: High mortality if untreated; Risk of massive PE and sudden death; Requires anticoagulation or thrombolysis; If indicating rheumatological disease: May indicate systemic disease exacerbation; Risk of progressive pulmonary dysfunction
Section 5: Follow-up Tests
- Recommended Additional Tests: Pleural Fluid Protein: Determines transudate vs exudate using Light's criteria; Essential for classification; Always ordered with LDH; Serum LDH: Measured on same day for LDH ratio calculation; Necessary for accurate interpretation; Helps confirm exudative process; Serum Protein and Albumin: Helps assess nutritional status; Used in Light's criteria; Important for diagnosis of transudative causes; Pleural Fluid Cell Count and Differential: Identifies inflammatory cell types; Helps narrow differential diagnosis; High lymphocytes suggest TB; High neutrophils suggest bacterial infection
- Pathogen/Disease-Specific Testing: Pleural fluid culture (bacterial, fungal, mycobacterial): If infection suspected; Pleural fluid glucose and pH: Indicators of bacterial infection or specific diseases; Pleural fluid ADA (adenosine deaminase): Highly suggestive of tuberculosis; Pleural fluid cytology: For suspected malignancy; Pleural fluid LDH isoenzymes: Sometimes used for specific diagnostic purposes; Tuberculosis-specific testing (TB culture, PCR, Xpert MTB/RIF): If TB suspected based on lymphocyte predominance; ANA, RF, and complement levels: For suspected autoimmune pleural disease
- Further Investigation Strategies: If diagnosis unclear after initial pleural fluid analysis: Repeat thoracentesis after 24-48 hours; Pleural biopsy (needle or thoracoscopic); Chest CT with contrast to evaluate lung parenchyma; Imaging recommendations: Chest X-ray or ultrasound for effusion size and loculation; CT chest if malignancy or PE suspected; Systemic workup: Echocardiography if heart failure suspected (elevated LDH with low protein); Liver function tests if cirrhosis suspected; Serum creatinine and urinalysis if nephrotic syndrome suspected
- Monitoring Frequency: Acute conditions (pneumonia, empyema): Monitor with repeat thoracentesis after 24-72 hours if inadequate response to treatment; LDH used to assess therapeutic efficacy; Malignant effusion: Repeat analysis after therapeutic intervention (pleurodesis); Monitor during chemotherapy; Used to assess treatment response; Tuberculosis: May repeat pleural studies to confirm diagnosis; Monitor response to anti-TB therapy; Usually single thoracentesis sufficient for diagnosis; Chronic conditions: Follow-up based on clinical course and treatment response; Repeat thoracentesis only if clinical deterioration or diagnostic uncertainty; Ongoing monitoring of pleural effusion size by imaging
Section 6: Fasting Required?
- Fasting Requirement: NO fasting is required for pleural fluid LDH testing. The test is performed on fluid obtained during thoracentesis, not blood serum. Fasting status does not affect pleural fluid LDH levels.
- Patient Preparation: Thoracentesis procedure requirements: Patient should be seated upright or in semi-recumbent position for procedure; Avoid eating or drinking 2-4 hours before procedure (standard pre-procedure NPO status); Full informed consent required with discussion of risks and benefits; Co-testing requirements: If serum LDH is also needed for ratio calculation: Fasting is NOT required for serum LDH; Can be drawn anytime, preferably same day as thoracentesis
- Special Instructions: Anticoagulation management: Continue anticoagulants (warfarin, DOACs) unless specifically instructed otherwise; INR may be checked before thoracentesis if significantly elevated; Aspirin and NSAIDs can usually continue; Discuss anticoagulation status with physician; Medications to notify provider about: Thrombolytics or fibrinolytics; Recent chemotherapy; Immunosuppressive medications; Corticosteroids in high doses
- Sample Collection Specifications: Collection tube: Plain sterile tube (no additives) for most pleural fluid studies; Minimum volume: 20-50 mL (typically 50 mL for comprehensive analysis); Processing: Send promptly to laboratory; Do NOT refrigerate unless delay expected; Keep at room temperature; Process within 4 hours for best accuracy; Specimen handling: Avoid hemolysis during collection; Prevent sample contamination; Protect from light if available; Label clearly with patient identification and date/time of collection

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