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Partial Thromboplastin Time, Activated (aPTT)

Test code(s) 763X

It is important to confirm that the aPTT is reproducibly prolonged. Falsely prolonged values can be caused by pre-analytic variables such as improper specimen collection (eg, an underfilled tube). For patients with vascular access lines, heparin contamination is common even when the line is flushed with saline.1

The strategies outlined below may help identify the etiology of the prolonged aPTT:

a.  Mixing study: this test may help guide the evaluation in the direction of either an inhibitor or a factor(s) deficiency.

b.  Specific factors and/or inhibitor panels (eg, a lupus anticoagulant evaluation): the selection of these tests is based on the results of other screening tests (eg, thrombin time and prothrombin time [PT]/international normalized ratio) and the patient’s symptoms (bleeding/bruising, thrombosis, or asymptomatic).

As outlined in Question 2, many hereditary or acquired conditions can cause a prolonged aPTT. In addition, elevation of factor VIII and fibrinogen (both acute phase reactants) can shorten the aPTT. Given the non-specificity of the aPTT, the recommended test for monitoring unfractionated heparin is the Heparin, Anti-Xa assay (test code 30292). This chromogenic method is not affected by factor levels or lupus anticoagulant.2

No, because low molecular weight heparin (LMWH) has a minimal impact on the aPTT. The recommended test for monitoring LMWH is the same as that used for UFH: the Heparin, Anti-Xa assay (test code 30292). Therapeutic ranges for LMWH are provided for both types of heparin on the final report.

The published ranges are for adults only. It is very challenging to obtain normal pediatric samples to establish a reference range. Due to the immature contact factors found in pre-adolescent children, the upper reference interval is approximately 10% higher (or 3-4 seconds).3



  1. Marlar RA, Cook J, Johnston M, et al. One-stage Prothrombin Time (PT) Test and Activated Partial Thromboplastin Time (APTT) Test; Approved Guideline-Second Edition. Wayne, PA: Clinical and Laboratory Standards Institute; 2008. CLSI document H47-A2.
  2. Price EA, Jin J, Nguyen HM, et al. Discordant aPTT and anti-Xa values and outcomes in hospitalized patients treated with intravenous unfractionated heparin. Ann Pharmacother.  2013;47:151-158.
  3. Monagle P, Barnes C, Ignjatovic V, et al. Developmental haemostasis. Impact for clinical haemostasis laboratories. Thromb Haemost. 2006;95:362-372.


This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.

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