How should feces (stool) samples be stored for the Fecal Occult Blood Test (FOBT) to maintain stability?

Fecal Sample Storage for FOBT Testing

For fecal occult blood test (FOBT) samples, specimens should ideally be submitted to the laboratory within 24 hours when stored at 4°C, or collected directly into buffer-containing tubes to prevent false-negative results from hemoglobin degradation at room temperature.

Optimal Storage Conditions

Immediate Processing Timeline

• Samples without buffer should be refrigerated at 4°C immediately after collection and processed within 24 hours to prevent hemoglobin degradation 1
• At room temperature (25-28°C), 36.7% of FOBT-positive samples converted to false-negative results after just 24 hours, and 56.7% after 48 hours when stored in buffer-free containers 1
• Even with refrigeration at 4°C, 33.3% of buffer-free samples showed false-negative conversion after 48 hours 1

Buffer-Containing Collection Systems

• Buffer tubes dramatically improve stability and minimize false-negative results during pre-analytical delays 1
• Only 10% of samples in buffer tubes at room temperature showed negative conversion after 48 hours, compared to 56.7% in buffer-free containers 1
• No specimens in buffer tubes stored at 4°C showed negative conversion after 48 hours 1

Long-Term Frozen Storage (For Research Purposes)

Freezing Protocols

• Samples can be stored at -80°C for extended periods (up to 11 years) with fairly stable hemoglobin measurements and diagnostic performance 2
• Hemoglobin concentrations remain strongly correlated (0.77-0.85) across multiple freeze-thaw cycles 2
• Diagnostic performance indicators including sensitivity and area under the curve remain similar even after prolonged frozen storage 2

Freeze-Thaw Considerations

• Multiple freeze-thaw cycles have minimal impact on FOBT performance when samples are stored at -80°C 2
• Some FIT systems (RIDASCREEN) showed slightly higher hemoglobin levels after the third freeze-thaw cycle, while others (OC-Sensor) showed slightly lower levels 2

Alternative Preservation Methods

Room-Temperature Preservation Options

• FOBT cards and RNAlater provide the highest stability for samples stored at ambient temperature for up to 4 days 3
• These methods are particularly useful when immediate refrigeration is not feasible 3
• 70% ethanol and dry swabs also provide reasonable preservation when samples are immediately frozen 3

Critical Clinical Pitfalls

Patient Education Requirements

• Patients using buffer-free containers must be explicitly instructed to submit specimens to the laboratory as soon as possible 1
• The risk of false-negative results increases dramatically with each day of delay at room temperature 1
• Clear instructions about immediate refrigeration (below 4°C) are essential when buffer tubes are not used 4

Temperature Control

• Avoid room temperature storage beyond 24 hours for buffer-free samples to prevent significant hemoglobin degradation 1
• Samples should be kept below 4°C during transport if processing will be delayed 4