Surprises in Coagulation Testing

Expires October 2025

William Nichols, M.D.

Coagulation testing is complex. Unexpected or unexplained prolongations of screening clotting time tests−such as the APTT (activated partial thromboplastin time) or PT (prothrombin time)–are commonly occurring events.

This case-based presentation introduces the diagnostic considerations for prolonged APTT and/or PT, including pathophysiologic classification (“SICKFAIL”) and laboratory and clinical approaches for evaluation.


William Nichols, M.D., is an Associate Professor of Medicine and Laboratory Medicine and a Consultant in the Division of Hematology and the Special Coagulation Laboratory at Mayo Clinic in Rochester.


Our speaker for this program is Dr. William Nichols, an Associate Professor of Medicine and Laboratory Medicine at Mayo Clinic, as well as a consultant in Hematology and the Special Coagulation Laboratory. Dr. Nichols presents a case study that demonstrates how pre- and postanalytic variables of the patient and blood samples can have confounding effects on the laboratory evaluation of coagulation disorders.Welcome to Mayo Medical Laboratories Hot Topics. These presentations provide short discussions of current topics and may be helpful to you in your practice.

Thank you for the introduction.

This case-based presentation introduces diagnostic considerations for prolonged APTT and/or PT coagulation test results, including pathophysiologic classification (“SICKFAIL”) and summarizing laboratory and clinical approaches for evaluation.


I have no disclosures.

Utilization Message

As you view this presentation, consider the following important points regarding testing for prolonged APTT and PT (Prolonged Clotting Times Profile)

  • How is the testing going to be used in your practice?
  • When should the tests be used?
  • How will results affect patient management?

Plasmatic Procoagulant Pathways and Tests

This slide outlines the intrinsic, extrinsic, and common procoagulant pathways–that are amplifying cascades of coagulation complexes (enzymes, cofactors, substrates, phospholipid, and calcium) –resulting in generation of thrombin and transformation of fibrinogen to fibrin, the endpoint that is detected by clotting time tests.

The APTT test is initiated by adding a contact activator to citrated plasma along with phospholipid and calcium – and reflects the intrinsic and common procoagulant pathways and components.

The PT test is initiated by adding tissue factor, phospholipid, and calcium to citrated plasma – and reflects the extrinsic and common procoagulant pathways and components.

The DRVVT (dilute Russell’s viper venom time) test assesses the common procoagulant pathway and is performed by adding diluted phospholipid and Russell’s viper venom to directly activate factor X, along with calcium, and is mainly used for detecting lupus anticoagulant (LA) inhibitors (along with the APTT test for detecting LA).

The Thrombin Time (TT) test assesses the final transformation of fibrinogen to fibrin and is performed by adding exogenous thrombin to citrated plasma (with or without calcium), and is mainly used to detect anticoagulants such as heparin or direct thrombin inhibitors.

Prolonged Clotting Times–Mechanisms

There are 4 basic causes of prolonged clotting time test results:

  • Coagulation Factor Deficiency or deficiencies
  • Coagulation Inhibition
  • Both can sometimes be present
  • Or Neither (spurious results)

A mixing test (of patient and normal plasma) is typically used for initial laboratory evaluation of prolonged clotting times, and requires interpretation.

Inhibition vs. Factor Deficiency: Mixing Patterns -> Interpretation

This graph shows APTT mixing study results for 2 patient plasmas: one with a coagulation inhibitor (such as a lupus anticoagulant) and one with coagulation factor deficiency (such as hemophilia A or B).

The vertical axis shows APTT test results in seconds, and the arrow notes the upper limit of the normal range.

The horizontal axis shows different mixtures of normal and patient plasmas.

Both of the patient plasmas show similar APTT prolongations (at the far right: no mixing or dilution).

Mixing plasmas 1:1 (50% normal and 50% patient) typically results in APTT correction into the normal reference range for deficient plasma, whereas the inhibitor plasma fails to correct into the normal reference range.

Plasmatic Coagulopathies: Pathophysiologic Classification - SICKFAIL

There are 8 general categories of causes of prolonged clotting time test results that can be recalled with the mnemonic “SICKFAIL”:

  • Spurious: There are many conditions that are very common pre-analytical causes of abnormal coagulation testing results – and the next slide will focus on this category.
  • Inhibitors: Are also common and include specific inhibitors against a coagulation factor, nonspecific inhibitors such as lupus anticoagulant (LA), and global inhibitors such as heparin or direct-acting anticoagulants.
  • Congenital/Hereditary factor deficiencies: Are relatively common (factors I, II, V, VII, X, VIII, IX, XI, XII, XIII and unnumbered proteins including von Willebrand factor (VWF), and usually occur individually, that is deficiency of only 1 factor or protein, and vary in severity and significance.
  • Vitamin K deficiency: Is relatively common such as in neonates and hospitalized individuals.
  • Factor deficiencies (acquired): Usually are multiple deficiencies (with patterns) or sometimes only a single factor deficiency may occur on an acquired basis.
  • Anticoagulants: Are increasingly ubiquitous and include warfarin, heparin and low-molecular-weight heparin, and direct-acting inhibitors of thrombin or coagulation factor Xa.
  • ICF (intravascular coagulation and fibrinolysis) or DIC (disseminated intravascular coagulation): There are many causes and manifestations of ICF/DIC including prolonged clotting time tests.
  • Liver disease: Can cause a typical pattern of abnormal coagulation test results with varying severity and significance.

Spurious Coagulopathies : Mainly PreAnalytical = sample or patient issues

Spurious coagulopathies usually reflect pre-analytical (pretesting) conditions of the patient and/or blood plasma sample, including the many potential conditions listed here:

  • Elevated Hematocrit and/or incomplete filling of evacuated collection tubes – can cause spurious prolongation of the APTT and/or PT (see the next slide with explanations).
  • Collecting and testing or submitting other than citrated plasma (“blue top tube”) samples such as EDTA-anticoagulated plasma or serum is another cause.
  • Anticoagulants present in the patient and/or plasma sample, such as heparin from patient treatment or in a “line draw” blood specimen, or other anticoagulant effects (warfarin or direct-acting anticoagulants), or effects of other therapy such as fibrinolytic therapy with tPA (tissue plasminogen activator ) or DDAVP (desmopressin), or transfusions (plasma, cryoprecipitate, factor concentrate) can all affect APTT and PT test results.
  • Many other conditions-including difficult venipuncture causing clotting in vitro; lipemia, hemolysis and/or icterus – which interfere with photo-optical testing; suboptimal sample processing conditions including improper temperatures (too warm, too cold or not cold enough), and/or improper centrifugation to deplete platelets before freezing plasma for subsequent testing – can all adversely affect APTT and/or PT test results.

Be aware (suspicious)!–these are not rare.

Normal Hematocrit vs. High Hematocrit

The normal hematocrit image on the left shows that the one-tenth volume of citrate anticoagulant (present in fixed amount in an evacuated blood collection tube) is distributed into the plasma – that comprises about 60% of the collected blood – and the citrate lowers the free calcium ion, thereby stopping plasma from clotting until the plasma is recalcified for clotting time testing.

The high or elevated hematocrit image on the right shows that the fixed one-tenth volume of citrate anticoagulant is distributed into a much smaller volume of plasma compared with the normal hematocrit sample, resulting in excessive lowering or complexing of calcium ion and “resistance to recalcification”–usually causing spuriously prolonged APTT and/or PT test results.

Prolonged APTT & PT Case–Clinical Information

Here is our case:  A 16-year-old male with cyanotic congenital heart disease (reflecting transposition of the great vessels) was to have cardiac bypass surgery performed the following day.

Preoperative lab testing showed:

  • Elevated hemoglobin and hematocrit (20.9 gm/dL and 63% respectively)
  • Normal platelet count
  • Prolonged PT and prolonged APTT

Preoperative Coag Screening: Now What?

Now what should we do? Should the patient proceed with heart surgery? (In other words – are the prolonged PT and APTT simply spurious test results, reflecting the high hematocrit?)

What evaluations should be performed? Should we obtain: A) More clinical information or history review?; B) More testing?; C) Both?

What are the most likely diagnoses?: A) Spuriously prolonged PT and APTT reflecting high hematocrit?; B) Are other causes possibly present?

Preoperative Coag Screening: Hemostatic History

In this case the initial evaluations included obtaining relevant clinical information, including bleeding history review:

  • At age 2 years the patient had a wound hematoma after heart surgery; by itself this is perhaps not abnormal.
  • At age 4 years, he cut his tongue and bled for days and was transfused; this is clearly and significantly abnormal.
  • At age 10 years, he had oozing after dental extractions; by itself perhaps not abnormal.
  • Recent cardiac catheterization was followed by oozing from groin puncture;  by itself perhaps not abnormal.
  • The patient led a relatively inactive life, reflecting heart disease; therefore, he had few hemostatic challenges.
  • The mother’s brother has a lifelong bleeding history. This is very suspicious for possible presence of an inherited bleeding disorder.
  • Questions:
  • Is this a normal vs. abnormal history? The constellation of bleeding events suggests it is likely that a bleeding disorder is present.
    • Is this person a mild, moderate, or severe bleeder? This would be a moderately severe bleeding history. Severe bleeding history would typically also include spontaneous bleeding events occurring without provocation.
  • Is the bleeding disorder hereditary? And if so, is it autosomal or X-linked? Yes, likely hereditary and could be either autosomal or x-linked, based on available information.

Preoperative Coag Screening: Special Coag Lab Testing with citrate-adjusted samples

Preoperative subsequent coag testing was performed using collection tubes with citrate anticoagulant content adjusted by Nomogram for the elevated hematocrit with these results.

  • The PT was normal on retesting (11.8 sec; normal 10-12 sec)
  • The APTT remained significantly prolonged (76 sec; normal 25-40 sec)
  • And the unactivated PTT was also prolonged (107 sec; normal 45-60 sec)
    • With mixing study demonstrating correction into the normal range (PTT 1:1 mix = 57 sec)
  • The thrombin time was normal, essentially excluding heparin effects (21 sec; normal 20-22 sec)

Preoperative Coag Screening Interpretation

Why was the PT initially prolonged, but subsequently normal? (It was spuriously prolonged reflecting high hematocrit and normal with subsequent citrate-adjusted plasma sample collection and testing.)
What is the most likely diagnosis for the prolonged APTT and PTT?

  1. The mixing study shows correction and suggests coagulation factor deficiency, rather than inhibition (such as from lupus anticoagulant: LA).
  2. Family history together with hemostatic history review raises possibility of X-linked Hemophilia A or B.

Additional testing? Yes, coagulation factor activity assays such as VIII, IX, XI, and probably factor XII would be appropriate.

Preoperative Coag Testing–Additional Testing

Additional testing demonstrated normal values for factor VIII and for factor XI, but factor IX was very low (3%), diagnostic of hemophilia B of moderate severity.

Preoperative Coag Screening–Diagnoses, Management

Diagnoses in this case are 2 with respect to hemostasis.

  • Spurious clotting times prolongations reflecting elevated hematocrit
  • Hemophilia B, moderately severe, also known as “Christmas Disease”

This individual was able to undergo cardiac surgery that was managed with factor IX infusions (the Case has previously been published as listed)

Subsequent DNA-based testing demonstrated a glutamic acid substitution at residue arginine 248 in the catalytic domain of factor IX responsible for hemophilia B.

The Mother also carries this mutation.

Plasmatic Coagulopathies–Pathophysiologic Classification = SICKFAIL

In Summary, Firstly, remember the 8 categories of causes for prolonged clotting times.

MML CTQ-2016

Secondly, recommendations for evaluating prolonged APTT or PTT include obtaining and reviewing relevant clinical information, if possible.

If indicated, consider repeating APTT and/or PT testing locally with careful attention to patient status and sample collection and processing conditions.

If prolonged APTT and/or PT persist and are unexplained, consider obtaining testing such as a Prolonged Clotting Time Profile, MML #83097 or PROCT.

PROCT / Prolonged Clotting Tine Profile Mayo/MML #83097

This algorithm depicts the Mayo/MML Prolonged Clotting Times Profile, with reflexive testing. The Profile testing starts with the APTT and PT, along with the DRVVT and thrombin time (TT), and also includes measurements of fibrinogen and fibrin D-dimer and soluble fibrin monomer complex.

Today we don’t have time to further discuss this algorithm, nor all the details of the “SICKFAIL” listing and all the causes of APTT or PT prolongation. Perhaps these will later be the subject of an MML Hot Topics presentation.

Algorithmic Approaches to Coagulopathy Testing: Case-Based Illustrations

Thank you Dr. Nichols.  If you enjoyed this case-based presentation please join our faculty and staff in San Diego on December 2nd for a symposium entitled Algorithmic Approaches to Coagulopathy Testing: Case-Based Illustrations, which precedes the 58th American Society of Hematology Annual Meeting. Thank you.

MCL Education

This post was developed by our Education and Technical Publications Team.