How Good is your INR? The Use of Certified Plasmas in PT/INR Testing. INR System. How Good is your INR? Outline

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1 How Good is your INR? The Use of Certified Plasmas in PT/INR Testing Dot Adcock, MD Esoterix Coagulation Mayo/NASCOLA Coagulation Testing Quality Conference April 25, 2007 How Good is your INR? The Use of Certified Plasmas in PT/INR Testing Outline Overview of INR system including limitations Highlight the new CLSI (NCCLS) document on PT/INR calibration Discuss potential impact of implementation of PT/INR verification and calibration INR System Introduced by WHO in 1983 to standardize reporting of PT results in patients on antivitamin K therapy (warfarin) Prior to this: recommended therapeutic range 2 to 2.5 X PT control American Heart Association

2 Warfarin Sodium Anti-vitamin K therapy One of the most commonly administered drugs in the US 21 million prescriptions written in 2003 One of the drugs most commonly associated with adverse events. Must be monitored due to the drug s narrow therapeutic window Under anticoagulation = clotting risk Over anticoagulation = bleeding risk INR System Accuracy of PT results is essential For the safety of individual patients on AVK therapy Avoid chronic over- or under- anticoagulation Optimize dosing of drug or antidote To improve applicability of guidelines Inter-laboratory Variability in INR Results College Am Pathologists (CAP) 1998 proficiency testing showed lack of consensus for reporting INR in 15% while lack of consensus in PT reporting occurred in only 4% European external quality assessment schemes have reported up to 20% variation 1998 Survey of 119 labs in North America revealed almost 17% variability in results* *Adcock DM, Johnston M. Thromb Haemost. 2002;87:74-9

3 CV (%) of INR Results %CV Series Test Plasma INR Value Adcock DM, Johnston M. Thromb Haemost. 2002;87:74-9 Pre-calibration INR Accuracy Test Sample True INR Value % Labs within 10% of true INR value Adcock DM, Johnston M. Thromb Haemost. 2002;87:74-9 INR System Major sources of variation in INR results Calculating the INR incorrectly Incorrect determination of PT Incorrect determination of MNPT Variation in ISI

4 Incorrect ISI Use The Philadelphia Hospital Experience 2001 ISI was changed in computer for more responsive reagent but old reagent was used Resulted in at least 3 deaths and dozens of over doses CDC issued a public health alert Incorrect Determination MNPT Inadequate number or inappropriate population Lack of analysis with new thromboplastin Lack of verification with new thromboplastin lot Use of arithmatic rather than geometric mean Arithmatic mean assumes normally distributed data Geometric mean: transform each data point to log value, determine mean of log values, transform to original units Variation in ISI All thromboplastins are assigned an ISI value ISI of IRP vs manufacturer labeled ISI vs local ISI WHO Method: Perform PTs on thromboplastin to be tested and IRP using 60 fresh stable anti-vitamin K plasmas and 20 normal plasmas by the tilt tube method; ISI determined from slope using orthogonal regression Imprecision in the calibration of the commercial reagent against the IRP or in house standard ISI of a thromboplastin is at least three steps from the IRP and 3-5% variation is allowed for each calibration step If CV for each step is 3%, CV of the ISI is sq root 27 = 5.2% If CV for each step is 5%, CV of the ISI is sq root 75 = 8.7%

5 PT Ratio At ISI = 1.0 At ISI = 1.4 At ISI = 2.0-5% + 5% -5% + 5% -5% + 5% Adcock DM, Duff S. Blood Coag Fibrin. 2000;11:583 Variation in ISI Inconsistency in citrate employed for ISI determination compared to that used locally ISI tends to be 10% lower when plasma collected into 3.8% and ISI determined in plasma collected in 3.2%* Local effect of coagulation instrument on ISI value Statistically significant variation in ISI between methods of clot detection Statistically significant variation in ISI between brands using the same method of clot detection Can occur with different instruments of the same make** * Duncan, EM. Thromb Haemost 1994;72:84. **Poller, L et al. AJCP 1995:103:358. Coagulometer Effect Reagent A Manufacturer ISI Locally Calibrated ISI B C D E Adcock DM, Johnston M. Thromb Haemost. 2002;87:74-9

6 INR System Generic ISI: ISI determined for a group of instruments that use the same method of end-point detection Thromboplastin/Instrument Specific ISI: ISI specific for a particular thromboplastin/instrument combination INR System Variation in CV of INR results between laboratories is potentially greater when: thromboplastin with a generic ISI is used vs thromboplastin with an instrument specific ISI Mean % difference between manufacturer ISI and locally calibrated ISI* Thromboplastin/instument specific ISI = 3% Generic ISI = 7% *Adcock DM, Johnston M. Thromb Haemost. 2002;87:74-9 Procedures for Verification of INR and Local Calibration of PT/INR Systems; Approved Guideline (July 2005) CLSI (NCCLS) H54-A

7 Optimization of the INR System Thromboplastin with an instrument specific ISI rather than a thromboplastin with a generic ISI Thromboplastin with a generic ISI INR verification is mandatory and PT/INR calibration strongly recommended Thromboplastin with instrument specific ISI INR verification is recommended and if this fails, PT/INR calibration Verification and Calibration Verification the process of proving that a procedure achieves the expected result and if not, the need for corrective action Calibration the process of testing and adjustment of a test system to provide a known relationship between a reference material and test sample Verification and Calibration Local application of the WHO calibration methodology not feasible 60 fresh AVK & 20 NP; IRP; tilt tube method Not enough IRP, labor intensive, costly, not enough expertise with tilt tube methodology Can be achieved using certified plasmas Normal or abnormal plasmas assigned a PT/INR value according to CLSI guideline (3 methods)

8 INR Verification Requires a minimum of 3 certified plasmas in the range of INR Tested in duplicate over two to three days Set of certified plasmas are evaluated in a manner typical of clinical samples PT/INR determined for each certified plasma INR obtained locally should compare to value assigned to the certified plasma by +/- 15% Frequency of INR Verification Required When a thromboplastin with generic ISI is used Recommended With a thromboplastin/instrument specific ISI reagent Following any change in reagent, lot #, instrument Following major instrument repair or service At least annually if no major changes occur Considered Following aberrant QC or PT result if cause for discrepancy not obvious INR Verification If verification fails Action must be taken Clinical results cannot be reported until discrepancy is addressed Make certain instrument is functioning properly, validation was performed properly and that no clerical errors were made Local calibration should be performed

9 PT/INR Calibration Prescribed number of certified plasmas are assayed Number of plasmas required depends on the nature of the certified plasma Must use different manufacturer or lot of certified plasma than was used for verification Certified Plasma Sources of Variation Source of plasma Immunodepleted or AVK Single donor or pooled plasmas Preparation of plasma Citrate concentration Frozen or lyophilized Assignment of PT/INR value PT/INR can be assigned against an IRP, against an in house standard or by consensus IRP used Certified Plasma Sources of Variation Procured from AVK plasma or artificially depleted of vitamin K factors Some PT reagents are more responsive to the presence of PIVKAs than others Single donor or pooled plasmas Fewer certified plasmas can be used if plasmas are pooled Reduces biologic variation

10 Certified Plasma Sources of Variation Type and concentration of anticoagulant Plasmapharesis bags - 4% citrate Frozen or lyophilized plasmas Freeze-drying may activate FVII Stabilizers added to lyophilized product may affect PT results Certified Plasma Sources of Variation PT/INR can be assigned against an IRP, against an in house standard or by consensus If the IRP is used: Human, rabbit and bovine IRPs like-to-like calibration should be observed PT/INR Calibration Based on all of these variations, a particular set of certified plasmas may not be appropriate for all thromboplastins Refer to package instrument of the certified plasma to see which thromboplastins it has been designed for or tested with

11 PT/INR Calibration Local calibration can be done to either: Calculate a local ISI Using certified plasmas and a modification of WHO method of assigning ISI Generate a local calibration line - Direct INR Using certified plasmas to establish a relationship between assigned INR and local PT PT/INR Calibration Calculation of local ISI PT of certified plasmas determined using the local system Duplicate testing over at least three days using the prescribed number of certified plasmas PT obtained locally is plotted ( Y axis) against the value assigned the certified PT (X axis) ISI is determined from the slope of the line using log scales and orthogonal regression analysis Ideally manufacturer will provide ISI calculation service PT/INR Calibration Preparation of direct INR calibration line PT of certified plasmas determined using the local system Duplicate testing over at least three days using the prescribed number of certified plasmas PT obtained locally (Y axis) is plotted against the value assigned to the certified plasmas (X axis) Best fit line determined using orthogonal regression analysis; log scales should be used R 2 must be > 0.95 to be valid Direct INR is independent of ISI and MNPT

12 Direct INR Calibration Line 100 Locally determined PT values PT (sec) Direct INR may be interpolated or calculated INR Value Certified Plasma Assigned INR Values Direct INR Calibration Line INR can be interpolated off calibration line using patient PT or the INR can be calculated algebraically Valid as long as QC remains within limits New line should be prepared with change in reagent, reagent lot, instrumentation of other major change in technique PT/INR Calibration Local system verification is required following local calibration Verification results should produce INR values within +/- 15 % of the assigned INR value of the certified plasma True value + (15% x true value) True value - (15% x true value)

13 PT/INR Verification and Calibration Will certified plasmas be FDA approved in the US and when might this happen? Can this process be implemented easily? Will this process be accepted? Will this process improve accuracy? Will this process improve patient outcomes? Evaluation of Frozen Plasma Calibrants for Enhanced Standardization of the INR: A Multi-center Study* 119 sites; 6 frozen certified plasmas Study demonstrated: Calibration procedure was simple and successfully completed in all laboratories Post calibration local ISI varied from manufacturer ISI by up to 14% Determination of improvements in accuracy was flawed by study design *Adcock DM, Johnston M. Thromb Haemost. 2002;87:74-9 Effect of local calibration on ISI Dade Behring Thromboplastin C+ IL Test PT Fibrinogen Organon T Simplastin L Ortho Recombiplastin Stago Neoplastine C1 + Range Man ISI Range LC ISI % Difference

14 UK Experience Data from Steve Kitchen, FIBMS, PhD In the UK more than 100 centres have routinely used local calibration for 7 years Currently 20 25% of laboratories perform PT calibration Three products available in Europe AK Calibrants Immuno/Technoclone MCR Manchester Reagents/Helena TRC Thrombosis Reference Center/Hart Biologicals Plasma Calibrants most commonly used by UK NEQAS participants Source Helena/Manchester Comparative Reagents (MCR) Technoclone AK Calibrants Hart Biologicals (formerly Thrombosis Reference Centre) Number Type of plasmas 20 Artificially depleted 4 1 Pooled normal 3 Pooled coumarin coumarin 1 normal Method by user 4 replicates orthogonal regression Local ISI Duplicate test Linear regression Local ISI or direct INR 4 replicates Local ISI Linear regression Summary of results N = 22 test plasmas Local Calibration Manufacturers ISI Mean INR Mean CV (%) Tech/ Immuno Helen a/mc R Hart/ TRC PT Fib HS Plus HTF Innovin Difference between groups 4%

15 PT/INR Calibration Published Results Long-term French experience (since 1977): Inter-laboratory CV decreased from 12% to 6% Canadian experience using direct INR: Inter-laboratory CV decreased from ~10% to 4% Italian experience: Inter-laboratory CV decreased from ~11% to 6% Inter-laboratory Variability in INR Results Data from CAP proficiency testing surveys* Variation in reported INR 1992: 18% Variation in reported INR 1998: 8% Variation in reported INR 2004: 6% Samples had mean INR Presented by Dr. John Olson Mayo QA Mtg 2005 Conclusion Despite the introduction and acceptance of INR in most laboratories, better standardization of PT/INR is needed The use of certified plasmas to validate and if necessary calibrate local systems may enhance standardization and ultimately improve patient outcomes