Frequency of Controls for PT and PTT Testing
Controls for prothrombin time (PT) and partial thromboplastin time (PTT) must be run daily and whenever there is a change in reagent batch or instrument, with additional testing required after any adjustments until stability is achieved.
Standard Control Requirements
PT and PTT are critical coagulation tests that require strict quality control measures to ensure accurate results. Based on current guidelines:
- Daily controls: Controls must be run at least once daily before patient testing begins 1
- Reagent changes: Controls must be run whenever there is a change in reagent batch 1
- Instrument changes: Controls must be run after any change or maintenance to the testing instrument 1
- After adjustments: When adjustments are made to testing parameters, controls should be repeated until stability is demonstrated 1
Additional Control Requirements
Beyond the standard daily controls, additional control testing is required in specific situations:
- After calibration: Controls should be run after any ISI calibration or INR determination 1
- Quality assessment: Regular quality assessment with certified plasmas should be performed to verify INR accuracy 1
- Multiple sessions: For ISI calibration, testing should be performed in quadruplicate in the same working session and repeated on three different days to control for day-to-day variation 1
- Direct INR determination: Testing of certified plasmas for direct INR determination should be performed in at least three separate sessions or days 1
Control Materials and Procedures
The type and handling of control materials are critical for accurate PT and PTT testing:
- Control materials: Use certified plasmas with assigned values for different reference thromboplastins 1
- Multiple levels: Controls should include normal, therapeutic, and critical ranges
- Storage conditions: Controls must be stored according to manufacturer recommendations, as improper storage can affect stability 2
- Processing time: For whole blood samples, PT testing is stable up to 24 hours and PTT testing up to 12 hours when transported at room temperature or 4°C 2
Monitoring and Documentation
Proper documentation of control results is essential:
- Record all control results: Document all control values with date, time, and operator
- Track trends: Monitor control values over time to detect shifts or trends before they affect patient results
- Corrective actions: Document any corrective actions taken when controls are out of range
Special Considerations
- Heparin monitoring: When monitoring heparin therapy, controls should be run with each batch of patient samples to ensure accurate aPTT results 3
- Warfarin monitoring: For INR monitoring, controls should be run daily and whenever PT reagents or instruments change 4
- Critical values: For patients with critical coagulation values, more frequent control testing may be warranted 1
Common Pitfalls to Avoid
- Inadequate mixing: Insufficient mixing of control materials can lead to erroneous results
- Improper storage: Control materials must be stored according to manufacturer specifications
- Expired reagents: Using expired reagents or controls can lead to inaccurate results
- Sample volume errors: Incorrect blood-to-anticoagulant ratios can significantly affect results, particularly for aPTT 5
- Delayed testing: Prolonged storage of samples can affect coagulation test results, with aPTT being more sensitive to storage conditions than PT 2
By adhering to these control requirements, laboratories can ensure the accuracy and reliability of PT and PTT testing, which is critical for proper diagnosis and management of coagulation disorders and monitoring of anticoagulant therapy.