Hematology: activated partial thromboplastin time (APTT) in Cats (Felis) | Vetlexicon
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Hematology: activated partial thromboplastin time (APTT)

ISSN 2398-2950

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Synonym(s): aPTT, PTT, Partial thromboplastin time, (Kaolin-cephalin clotting time)

Overview

  • Evaluating intrinsic (XII, XI, IX, VIII) plus common (X, V, II, I) pathways help determine whether bleeding is due to one or more defects in coagulation (secondary hemostasis) from other causes (eg trauma, ulceration, neoplasia). 

Uses

Alone

  • Evaluation of intrinsic (surface induced) and common pathways in the clotting cascade. 

In combination

Other points

  • Half-lives for the various factors vary with mammalian species and are unknown exactly but from human extrapolation, likely of the order of hours (factor VII) to days, averaging 1-2 days.  
  • Functional detection of deficiencies in factors VIII and IX Hemophilia more commonly and clinically significant, even fatal. However XII also, with XI, X, V, II, I and/or prekallikrein are being evaluated. Individual deficiencies are estimated though to be <30% for detection (ie prolongation in the time). 
  • Alternatively but more crudely assessed via whole blood clotting time or activated clotting time  Hematology: activated clotting time, but levels of clotting factor then estimated at <5-10% normal. 
  • More dynamic testing (eg thromboelastography Thromboelastography (TEG)) is now, albeit less widely, available and more accurately describes the clotting cascade including hypercoagulable states.  
  • If APTT is significantly prolonged, specific factor assaying can then be performed (sequentially) using breed reported risk or commoner deficiencies initially (eg factor VIII / anti-hemophilic factor).  
  • Inherited factor deficiency can be sex-linked, the mutation found on the X chromosome (eg VIII, IX) and so a male phenotypic disease. Others (eg II, VII, XII are more complex autosomal diseases with variable penetrance Hemostatic disorders: inherited.  
  • Separate genetic mutational tests are available for many inherited traits in specific breeds where the genetic cause/location is known. 

Sampling

Source of test material

  • Anti-coagulated whole blood. 
  • Standard venipuncture Jugular venipuncture
  • Do not collect samples from veins with an (indwelling) catheter or from the catheter itself. 

Quantity of test material

  • Typically 3.2% (0.109 mol/L), or less commonly 3.8% (0.129 mol/L), anti-coagulated trisodium citrate tubes (often green or blue top) filled correctly (to the indicated line). 
  • At least 1 ml whole blood is recommended. This typically yields about 0.5 ml of plasma, allowing for standard hemostatic testing and any required duplicate testing.  
  • If APTT is prolonged and specific factor testing is required, repeated fresh sampling is typically recommended. Alternatively, the initial volume of blood can be doubled to allow for possible additional plasma measurements, presuming test stability allows. 

Sample collection technique

  • Must be quick and clean venipuncture with smooth blood flow. 
  • Using a short (<3") butterfly catheter may prevent repeated venipunctures in fractious animals, especially for multiple blood sample tubes. 
  • Using closed sample collection systems with a liquid anti-coagulated vacutainer (if available or feasible) is ideal. This starts anti-coagulating the blood as it's collected. 
  • Mix blood with anticoagulant otherwise as soon as the specimen is collected.  
  • Mixing is gentle, inversion and rolling (‘figure of eight’) for at least 10 seconds.  

Quality control

Precautions

  • Always fill anti-coagulated sample tube/vacutainer to its correct level. 
  • If open system sampling, ensure rapid transfer to citrated tube for gentle mixing.  
  • If a clot is present, that sample should not be submitted, the blood instead redrawn from a different vein. 
  • Once mixed and if plasma (cf. whole blood) is required for analyzer testing, centrifuge immediately (within 1 hour) at standard high speed (eg 10 min at 1500g) to separate plasma. Remove plasma into a separate labeled “Citrated plasma” plain tube. 
  • Some tests may be significantly affected by hemolysis, the effect though quite variable due to possible cell activation in vitro (eg platelet fragments). Hemolysis should ideally be avoided and a fresh sample redrawn from a different vein, but this can be checked with the relevant laboratory regarding suitability, if known and required. 
  • If the PCV/HCT varies significantly, relative differences in citration also impact on measured times (eg especially APTT). This includes anemic patients (eg HCT <15%, undercitration →shortened times) or those with higher/increased values (eg HCT >65%, overcitration → prolonged times). Limited veterinary studies but altered ratios may be recommended if noted, especially with erythrocytosis. Use the formula C (citrate vol.) = 0.002 x (100-HCT) x V (vol. blood) to correct.  

Timing of test

  • Standard fasted sampling recommended to avoid potential lipemic interference (especially automated optical methods). 
  • Varied storage temperature and whole blood or plasma effects on different hemostatic assays such as APTT.  
  • If whole blood, ideally <2  h. Ideally < 4 h if separated plasma.  
  • Most testing in separated plasma though is fine at room temperature for 24 h and if chilled for up to 48 h.   
  • Frozen (-20°C) clear plasma is stable for <3 months for many parameters including APTT, longer (c.18 months) for other parameters if deep frozen (-70°C). 

Sample transport

  • Ideally submit both tubes: the labeled citrated plasma and remaining blood cells to allow sample quality checking at the referral lab. 
  • If frozen (-20°C) to prolong stability (>48-72 h), plasma should be transported on ice to remain frozen and avoid freeze-thaw cycling as this degrades clotting factors for artifactual in vitro prolongation. 
  • Package according to Post Office regulations Transportation of diagnostic specimen
  • Do not send to laboratory when postal delay is anticipated (eg bank holidays, over weekend) or temperature extremes are expected (especially if not chilled or frozen). 

Tests

Methodologies

Manual, optical, electrical or electromagnetic  

  • Citrated plasma is incubated with surface activator (eg Kaolin) and procoagulant phospholipids (eg cephaloplastins, partial thromboplastin) at 37°C (eg water bath, heating block or cuvette/cup). 
  • Calcium ions (factor IV) are added (counter-acting the citrate chelation) and the solutions-plasma mixed. 
  • Time taken for fibrin to then form is measured (eg visible fibrin/gel-clot, light transmission (spectro-) or scatter (nephro-), electrode voltage difference, oscillating magnetized ball). 
  • Testing (especially manual) is often performed in duplicate with an average value reported.  

Availability

  • Varied bench top automated analyzers available for in house (mainly PT, APTT and/or ACT). 
  • Most clinical pathology laboratories offer a basic hemostatic/clotting profile  Hematology: complete blood count (CBC), coagulation times (PT and APTT) and possibly fibrin degradation products (FDP) Fibrin degradation products or D-dimers Hematology: D-dimers.   
  • Individual factor assays typically require sending to specific referral labs. 

Validity

Sensitivity

  • More sensitive than activated clotting time (ACT) Hematology: activated clotting time
  • Still relatively insensitive since activity of clotting factor must be <30% to result in abnormal elevation. However, this may correlate well with clinically evident bleeding. 

Specificity

  • Does not require presence of blood’s cellular components (eg platelets). 
  • Varies with method and potential in vitro factors as listed.  

Technique intrinsic limitations

  • Some manual methods or automated instrument platforms are designed for human testing, so less suitable for small animal specimens (faster clot formation relatively). The instrument and method should be validated for each specific species and specimens. 
  • Reference intervals are often poorly defined and data on healthy sampling conditions may not translate to patients with systemic diseases or if on anti-coagulant therapy.  

Result Data

Normal (reference) values

  • 10-20 seconds typically but quoted (decimal) values vary with method (manual or automated). 
  • (Can be quoted as % relative to <1.25x pooled plasma control specimen.) 
  • (Submission of a control specimen from a healthy cat may be helpful in excluding pre-analytical (in vitro) factors, more so when shipping internationally or for rarer factor assays.) 

Abnormal values

  • Traditionally (and somewhat historically) >10-20% prolongation is quoted to support a significant prolongation, especially with varied HCTs and in vitro effects.  
  • >20-25 seconds in many assays (or simply greater than the Upper Reference Limit) is prolonged significantly, supporting a coagulation factor abnormality/deficiency. 

Prolonged APTT 

  • The commonest deficiency is factor XII. Whilst this causes in vitro prolongation of APTT, it is only seen with clinical bleeding if associated with other factors, such as trauma, or deficiencies. 
  • Other factor deficiencies: commonly IX (eg Birman Birman), and XI with I (eg Maine Coon Maine Coon) less so X, or II (Fibrinogen deficiency). 
  • Prekallikrein deficiency (rare). 
  • Anticoagulant inhibition (eg warfarin, heparin therapy) but accurate figures, such as 1.5-2 times baseline, are more variable and do not necessarily correlate with targeted therapy. 
  • To support DIC Disseminated intravascular coagulation. APTT prolongation (with PT) in combination with reduced platelets Thrombocytopenia, RBC fragmentation and increased D dimers/FDPs. 
  • Liver diseases (such as lipidosis and cholestasis, more so with reduced hepatic synthesis of factors II, VII, IX and X. PT prolongation too and in theory prolongs earlier than APTT. Complex and changing dynamic between factor production, pro-coagulant generation and anti-coagulant clearance.  
  • Rodenticide anti-coagulant toxicity (eg coumarins - warfarin) Anticoagulant rodenticide poisoning due mainly from inhibited vitamin K dependent carboxylation (activation) of factors, II, VII, IX and X. 
  • (Severe) von Willebrand's disease (vWD) may result in slight prolongation of APTT (reduced stability and half-life of circulating factor VIII). 
  • Hereditary, often single factor (eg Siamese Siamese, British shorthair) but can be multiple (eg Devon Rex with vitamin K responsive coagulopathy Vitamin K responsive coagulopathy (Devon Rex)Hemostatic disorders: inherited.
  • Dilutional due to massive transfusion of plasma poor products (eg packed RBCs, crystalloids).  
  • (GI disease resulting in reduced vitamin K absorption (eg biliary obstruction, infiltrative mural pathology). 

Shortened APTT 

  • In theory, a shortened APTT can be seen with hypercoagulable states (eg glomerulonephropathy) but times appear too crude or variable for this reliably. Dynamic testing is more reliable, such as platelet aggregometry and thromboelastography.

Errors and artifacts

  • In vitro factors principally (eg over or under citration due to incorrect filling, sample tubes out of date, relative alterations in plasma-cellular volumes).  
  • Lipemic and hemolyzed samples, activating the cascade in vitro or affecting optical measurements 
  • Test is inhibited by heparin (catheter flushing and medicated therapy, eg LMWH Heparin). 

Further Reading

Publications

Refereed papers

  • Recent references from VetMedResource and PubMed.
  • Couto C J (1999) Clinical approach to bleeding dog or cat. Vet Med May, 450-459.
  • Lisciandro C L, Hohenhaus A, Brooks M (1998) Coagulation Abnormalities in 22 Cats with Naturally Occurring Liver Disease. JVIM 12, 71; 71-75 Wiley 
  • Green R A, White E  (1977) Feline  factor  XI1 (Hageman) deficiency. Am J Vet Res 38, 893-895. 

Other sources of information

  • Kaneko J J, Harvey J W & Brass M L (1997) Eds. Clinical Biochemistry of Domestic Animals. 5th edn. Academic Press, USA.
  • Jain N C (1993) Essentials of Veterinary Hematology. Lea & Febiger, USA.
  • http://eclinpath.com/hemostasis/tests/.