POC Coagulation Testing

by Donna Castellone • January 16, 2024

The interpretations below are provided by Donna Castellone, MS, MT (ASCP) SH for Aniara Diagnostica.

Introduction

Coagulation testing takes time. The sample has to be collected, sent to the laboratory, spun for platelet poor plasma, placed on a line or on an analyzer, resulted and then verified. One of the most important things in the hospital is length of stay. Laboratories are tasked with their contribution which means decreasing the turn around time to provide clinicians with results to determine the status of the patient and if any adjustments in anticoagulation is needed. Compounding this situation is staffing in the laboratory. When was the last time there was a full staff in your laboratory? A solution is to utilize point of care testing (POCT). Using POCT is a powerful tool that can provide quality results with rapid turn-around times. POCT has become one of the fastest growths in the medical field with an increase of 10-12% annually.(1) The ability of the laboratory to oversee high quality coagulation results has enabled this rapid testing to be performed in the emergency department, operating room, clinics and intensive care units.(2)

POCT in coagulation includes: Prothrombin Time (PT) and International Normalized Ratio (INR), Activated Clotting Time (ACT), Activated Partial Thromboplastin Time (aPTT), D-dimer (DD),and testing for anti-platelet drugs. However, it is important to understand how these tests compare to your laboratory testing, which is truth. It is important for POC testing to be verified against laboratory testing and understand if there are any limitations.

Prothrombin Time and INR

Both laboratory and POC PT/INR systems demonstrate variability as the INRs increase. This is further confounded when thromboplastins are derived from different sources (human versus rabbit).(3) POC analyzers also have different methods to determine clot formation including optical, electro-mechanical and electrochemical clot detection. It is important for the POC analyzers to be compared to the laboratory analyzers across the measuring range to understand at what point the INR correlate with the analyzer and the patient's clinical condition.(4) In order to minimize discrepancies between the POC analyzer and the laboratory analyzer, ideally both analyzers should be standardized using the same type of thromboplastin reagent (eg. Recombinant to recombinant).

Several factors will increase the POC INR results. These include hematocrit and protein induced vitamin K absence (PIVKA). INRs may also differ if a patient is not in a steady state of anticoagulation, which usually occurs within 5 weeks from the time the patient started. When a patient has a low hematocrit there will be excess plasma for the amount of cartridge in the POC analyzer. This results in slowing down clot formation resulting in a higher INR. The presence of PIVKA does not inhibit clotting in the body, but they do inhibit the thromboplastin used in the INR test making the INR appear elevated.(5)

There is inherent bias between the laboratory INR and the POC INR when the INR measurement is elevated above 3. It is important to know at what INR the POC analyzer discrepancy occurs. When levels of VKA are supra therapeutic (>5) or when the anticoagulation therapy is not stable, INR discrepancies are exacerbated. In guidelines for patient INR self-monitoring it is recommended that any INR between 4-8 should be repeated on a laboratory analyzer.(6)

A study that looked at the agreement between the POC analyzer and the laboratory instrument in 200 samples (CoaguChek XS Pro versus the Sysmex CS2000i) demonstrated that the correlation was different in different INR ranges and increased as the INR results increased. The mean difference was 0.09 in the subtherapeutic range (<1.9 INR), 0.29 INR is the therapeutic range (2.0-3.0 INR), and 0.4 in the supratherapeutic range (>3.0). There is a positive bias in the upper range of the INR which could impact dosing and should be confirmed by a laboratory INR measurement.(7) Advantages of using a POC INR include shorter time to result, decreased need for venipuncture, greater time in therapeutic range and better dose adjustment leading to patient safety.(6)

Activated Partial Thromboplastin Time

Laboratory aPTT reagents have varying sensitivities based on the activator as well as the source of phospholipids. POC aPTT reagents also have varying sensitivities to heparin, lupus anticoagulants and has had variable results in the evaluation of a coagulopathy.(6) The POC aPTT can also be affected by the platelet count and the hematocrit.(3)

POC aPTT can be used to measure low dose heparin. This methodology demonstrates a linearity of up to approximately 1.0U/mL unfractionated heparin (UFH). When comparing the results of POC aPTT to laboratory aPTT there have been variable results.(6) A study that looked at 390 patients in acute and intensive care settings reported an acceptable correlation between laboratory versus POC aPTT. While a prospective study of patients on heparin in the surgical intensive care, showed poor agreement between POC versus laboratory aPTT as well as a poor correlation with anti-Xa activity. Other studies have shown a poor correlation in cardiac surgery patients on heparin between the two tests. As a result, this study suggested the test should not be used on this cohort of patients.(7)

The utility of a POC aPTT is limited. It can be used for the evaluation of bridging heparin to vitamin k antagonists, assessment of pre and post transfusion hemostasis as well as monitoring low-dose heparin. An advantage in using a POC aPTT may reduce the time needed to achieve a therapeutic level when using low dose heparin therapy.(6)

Activated Clotting Time

The Activated clotting time (ACT) is a POC coagulation test that is used to measure the anticoagulant effects of UFH, but not the heparin concentration. The ACT measures the time of clot formation through the intrinsic pathway using activators of factor XII. These include celite, kaolin, glass beads and ellagic acid. There is an increasing linear relationship in the result of the ACT to the amount of heparin that the patient has received.(8) This POC test is used to monitor patients receiving low, moderate, or intensive anticoagulation in invasive or operative clinical procedures that use heparin therapy. The ACT may be beneficial to determine heparin levels in procedures such as: dialysis, cardiac catherization, angiography, extracorporeal membrane oxygenation, and valve replacement.(9)

The ACT has a wide analytical measuring range since the required level of heparin anticoagulation will vary depending on the clinical procedure. An analyzer with an analytical range of 68-400s may be sensitive up to 2.5 IU/mL of heparin, whereas an analyzer with a range of 65-1005 s can detect levels up to 6 IU/mL of heparin. (10) IT is not sensitive to low molecular weight heparin.

As a result, these variables may influence ACT analytical performance characteristics. Additionally, the ACT has no "gold standard" or laboratory equivalent that can be used as a reference measure which makes it difficult to standardize ACT methods and results will differ using different POC instruments. It is important for testing sites to use the same device when evaluating patients to ensure results reflect patient heparin concentration and not differences in test systems.(3)

D-Dimer

An elevated D-dimer can indicate many conditions including venous thromboembolism (VTE) or the development of disseminated intravascular coagulation (DIC) but is not specific to these conditions. Elevation can also be seen in infection, inflammation, myocardial infarction and malignant neoplasms. The validity of the test lies in its negative predictive value and its ability to rule out the presence of a VTE. Several POC D-dimer tests have the ability to determine the exclusion of VTE, while others can only claim it as a tool in the aid in diagnosis. It is important to know which type of POC test is being used. There are currently two POC D-dimer assays that are FDA approved with exclusionary claims: Siemens Stratus D-dimer, and Roche cobas h 232 POC system. It is important to understand the characteristics of the D-dimer methodology, the sensitivity, negative predictive value as well as precision at the cut off level.(4)

Laboratory based quantitative assays have higher sensitivity compared with POC testing and may be a better evaluation to exclude pulmonary embolism. Several recent studies have demonstrated that in a very low risk emergency population such as primary care patients, along with the Wells clinical scoring system a qualitative POC test has a similar accuracy as the laboratory quantitative D-dimer.(11)

Guidelines from the National Institute for Health and Care Excellence (NICE) state that if a D-dimer result cannot be obtained within 4 hours, anticoagulation should be given in patients suspected of VTE. This may needlessly put a patient at risk for bleeding. A study that compared the use of a POC D-dimer saw a decrease in time to diagnosis and treatment to 1 hour versus 4-6 hours from a laboratory D-dimer. There were less patients who received interim anticoagulation.(12)

Platelet Testing

POC analyzers using whole blood have been developed to detect anti-platelet drugs including cyclooxygenase inhibitors (e.g. aspirin), P2Y12 antagonists such as thienopyridines (e.g. clopidogrel, prasugrel, ticagrelor), and GPIIb/IIIa (glycoprotein IIb/IIIa) antagonists (e.g. abciximab, tirofiban, eptifibatide).(13) It is important to determine pre-surgery if these drugs are impacting platelet aggregation and may result in post-operative bleeding. Several systems have been implemented for platelet testing and they are POC analyzers since they fill the requirement of near patient testing, however some of the methods are not waived and require testing by qualified laboratory personnel.

The VerifyNow system uses a turbidimetry-based optical detection that measures platelet-induced aggregation using fibrinogen coated beads. This is a cartridge-based system containing specific agonists to detect specific drugs (aspirin, Plavix and abcixmab) using specific agonists. Whole blood must be used within 4 hours and kept at room temperature. Results are expressed as aspirin reaction units (ARU). The cutoff is 550 ARU.(14)

The Platelet Function Analyzer (PFA 100) measures platelet adhesion and aggregation under high shear stress using citrated whole blood. Cartridges containing epinephrine (EPI) and collagen(COL)and ADP and collagen are used and the amount of closure time it takes for a platelet plug to occlude the aperture is measured in seconds. The EPI/COL cartridge is prolonged in the presence of aspirin and the ADP/COL is normal. When both cartridges are prolonged a further evaluation of primary hemostasis should be conducted. Other antiplatelet drugs have not yet been able to be evaluated by this method.(15)

Recommendations for POC Coagulation testing

The following guidelines provide recommendations for POC coagulation testing.

  1. A new point of care (POC) device should be assessed for reproducibility against a central laboratory analyser or other established technology.
  2. The sensitivity and specificity of the D-dimer test method should be established such that, when combined with the pre-test probability score, the test can be used as a test of exclusion for venous thromboembolism or as a diagnostic tool used in conjunction with diagnostic imaging.
  3. Users should be aware of the method employed by a point of care testing (POCT) device to measure parameters and the possible limitations of the method.
  4. International normalised ratio POCT methods should be assessed for comparability with central laboratory analysers using warfarinsed samples in order to establish a reflex testing algorithm for confirmation of supra-therapeutic levels.
  5. The activated clotting time (ACT) should be used for monitoring high doses of heparin where standard activated partial thromboplastin time (aPTT) methods are insensitive.
  6. POC aPTT measurement should not be used for monitoring heparin unless the method has been shown to correlate with an established proven method.
  7. In the absence of an established direct oral anticoagulant (DOAC) assay, thromboelastography (TEG) and ACT, once verified, can be used to detect dabigatran-induced coagulopathy and guide the reversal of same, and the TEG 6s for detection and classification of DOACs.(4)

Conclusion

The implementation of POC coagulation testing provides timely results that can improve turn around time and decrease the time to treatment. It is important that POC analyzers are correlated to laboratory analyzers to ensure consistent results. Quality control ensures that testing meets standards and analyzers are performing within specified criteria. This is also seen in POC testing. POC may be more costly, but the downstream effect may allow decreased length of stay. With the constraints within hospitals and the urgency to move patients along, POC testing in coagulation will keep increasing and expanding.

References:

  1. Murray, Patrick, PhD. "Laboratory Strategies: Meeting the Increasing Demand for Point-of-care Diagnostics." MLO. October 2014. Accessed at: https://www.mlo-online.com/home/article/13007451/laboratory-strategies-meeting-the-increasing-demand-for-pointofcare-diagnostics
  2. Clifford, LJ., The pros and cons of point-of-care testing vs laboratory testing, MLO,The pros and cons of point-of-online.com), 2018.
  3. CLSI. Point-of-Care Coagulation Testing and Anticoagulation Monitoring. 2nd ed. CLSI guideline POCT14. Clinical and Laboratory Standards Institute; 2020.
  4. Mooney, C., Byrne, M., Kapuya, B., Pentony, L., De la Salle, B., Cambridge, T., Goley, C., Point of care testing in general haematology on behalf of the British Society for Haematology Guideline, British Journal of Haematology, 2019, 187, 296–306.
  5. Anthony, L. Point-of-Care INR Testing: Limitations that Impact Clinical Performance and Utility POCINR limitations.pdf March 2013
  6. Wool, GD., Benefits and Pitfalls of Point-of-Care Coagulation Testing for Anticoagulation Management: An ACLPS Critical Review, American Journal of Clinical Pathology, 2019,151:1-17.
  7. Moiz,B., Rashid, Hassan, M., Prospective Comparison of Point-of-Care Device and Standard Analyzer for Monitoring of International Normalized Ratio in Outpatient Oral Anticoagulant Clinic, Clinical Applied Thrombosis And Hemostasis, 2018.
  8. McPherson RA. Matthew R, Pincus, MR, clinical diagnosis and management by laboratory method 22nd ed. Philadelphia (PA), Elsevier Saunders 2011.
  9. Korpi-Steiner, NL., Walz, JM., Schanzer,A., Lokinendi V, Comparison of Point-of-Care Activated Clotting Time Methods in Different Clinical Settings in a Large Academic Medical Center. The Journal of Applied Laboratory Medicine 2019.
  10. Toben, B., Martin, M., Rapid Assessment of Coagulation at the Point of Care With the Hemochron Signature Elite System, Point of Care: The Journal of Near-Patient Testing & Technology: 2020, 19:116-121.
  11. Runyon MS, Beam DM, King MC, Lipford EH, Kline JA. Comparison of the Simplify D‐dimer assay performed at the bedside with a laboratory-based quantitative D-dimer assay for the diagnosis of pulmonary embolism in a low prevalence emergency department population. Emerg Med J 2008; 25: 70– 5.
  12. Implementing point-of-care D-dimer tests for deep vein thrombosis (DVT) NICE GUIDANCE December 2020 Implementing point-of-care D-dimer tests for deep vein thrombosis (DVT) | NICE
  13. Gibbs NM. Point-of-care assessment of antiplatelet agents in the perioperative period: a review. Anaesth Intensive Care 2009; 37: 354–69
  14. Paniccia, R., Antonucci, E., Gori, AM., Marcucci, R., Poli, S et al Comparison of Different Methods to Evaluate the Effect of Aspirin on Platelet Function in High-Risk Patients With Ischemic Heart Disease Receiving Dual Antiplatelet Treatment, American J Clinical Pathology, 2007;128:143-149.

« January 2024: Events Happening in February 2024
January 2024: New In Coagulation »