How many days can pleural fluid be stored in a fridge for diagnostic purposes?

Pleural Fluid Storage Duration in Refrigerator

Pleural fluid samples should be processed immediately after collection, but if delayed, most biochemical tests remain stable for up to 4 days when refrigerated at 4°C, with the critical exception of LDH which degrades after 2 days and glucose which is unstable throughout. 1

Optimal Processing Timeline

• Immediate processing is ideal for all pleural fluid analyses to ensure maximum diagnostic accuracy 1
• Refrigeration at 4°C preserves most analytes for 4 days, including protein, albumin, cholesterol, and triglycerides 1
• Room temperature storage (21°C) also maintains stability for up to 4 days for most parameters except glucose and LDH 1

Critical Time-Sensitive Parameters

LDH (Lactate Dehydrogenase)

• LDH becomes unreliable after 2 days at 4°C due to instability of isoenzymes 4 and 5 1
• Freezing at -20°C causes LDH degradation within 24 hours 1
• This is particularly important since LDH is essential for Light's criteria to differentiate transudates from exudates 2

Glucose

• Glucose measurements are unstable throughout storage at any temperature 1
• Should be analyzed immediately if glucose levels are clinically relevant 1

Microbiological Samples

• Pleural fluid for culture should be processed within 24 hours in suspected pleural infection 2
• Send 5-10 mL in blood culture bottles (aerobic and anaerobic) plus sterile containers 2
• Refrigeration at 4°C for up to 12 hours is acceptable if immediate processing is not possible 3
• Never freeze samples intended for culture as this damages bacterial cell membranes and reduces viability 3

Cytology Samples

• 25-50 mL should be submitted for cytological examination when malignancy is suspected 2
• Cytology samples are generally stable refrigerated, though processing within 24-48 hours is preferred for optimal cell morphology 4

Practical Storage Algorithm

For samples requiring delayed processing:

1. Within 1 hour: Transport at 4°C (on ice) 3
2. 1-4 days: Refrigerate at 4°C for protein, albumin, cholesterol, triglycerides 1
3. Up to 2 days only: Refrigerate at 4°C for LDH analysis 1
4. Up to 12 hours: Refrigerate at 4°C for microbiological cultures 3
5. Immediate analysis required: Glucose measurements 1

Common Pitfalls to Avoid

• Do not freeze pleural fluid samples as this destroys LDH activity within 24 hours and damages bacterial viability 3, 1
• Do not rely on LDH results from samples stored >2 days at 4°C, as sensitivity for exudate classification drops significantly 1
• Do not delay microbiological processing beyond 12 hours even with refrigeration, as this reduces recovery of fastidious organisms 3
• Do not store samples at room temperature beyond 4 days for any biochemical analysis 1