Annals of Cardiac Anaesthesia Annals of Cardiac Anaesthesia Annals of Cardiac Anaesthesia
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Table of Contents
Year : 2013  |  Volume : 16  |  Issue : 2  |  Page : 155-156
Activated clotting time tube malfunction: A rare cause of heparin overdose

Department of Anaesthesia and Intensive Care, Post Graduate Institute of Medical Education and Research, Chandigarh, India

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Date of Web Publication29-Mar-2013

How to cite this article:
Kumar B, Badamali AK. Activated clotting time tube malfunction: A rare cause of heparin overdose. Ann Card Anaesth 2013;16:155-6

How to cite this URL:
Kumar B, Badamali AK. Activated clotting time tube malfunction: A rare cause of heparin overdose. Ann Card Anaesth [serial online] 2013 [cited 2021 Oct 25];16:155-6. Available from:

The Editor,

The activated clotting time (ACT) first described by Hattersley is routinely used as point of care test to monitor the anticoagulant effect of unfractionated heparin (UFH) in vascular surgery and during cardiopulmonary bypass. [1] Although there can be multiple causes for low ACT despite administration of recommended dose of heparin, it is commonly ascribed to inter individual variation in anticoagulant response to heparin, erratic behaviors of UFH among its own batches and low level of antithrombin III in the pediatric age group. [2] We faced a situation of low ACT in a child undergoing Blalock-Taussig (BT) shunt and later found a very unusual cause for this.

A 2-year old male child weighing 15 kg, with tetralogy of Fallot was scheduled for emergency BT shunt for repeated cyanotic spells. His preoperative hematology showed high hematocrit (60%), low platelet count (80,000/dL) and low prothrombin index (65%). The baseline ACT was 140 s. Induction and maintenance of anesthesia was uneventful. As per our institute protocol 1 mg/kg of UFH was administered through central line after verifying free back flow of blood before clamping of subclavian artery. ACT after 4 min of heparin administration showed a value of 160 s. A supplemental dose of UFH 1 mg/kg was administered. When the previous ACT tube (Actalyke C-ACT) which showed ACT 160 s was removed from the well of ACT monitor (Actalyke-Mini II, Helena Laboratories, Texas; US) to insert new ACT tube with fresh sample, no clot was found in the previous ACT tube. Careful examination of the ACT tube showed an unusual object inside the tube. The flip top of the ACT tube was opened and remaining blood was washed and we found two magnets inside the tube [Figure 1]. The ACT after a supplemental dose of UFH showed value of 700s. Patient required prolonged surgical hemostasis time before closure of chest and showed high chest drain output during the post-operative period that was managed conservatively.
Figure 1: Double magnet (arrow) in a activated clotting time test tube after washing out blood from the tube

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In the original Hattersley's method of measurement of ACT, the test tube was placed in a water bath at 37°C for 60 s, removed every 5 s and tilted slowly until a clot had formed. The time taken for clot formation was noted as the ACT. [1] Applying the same principle, now test is performed with a fully automated technique where the clot formation is recorded electronically. [3] Commercially available ACT test tube contains inert silicious earth (diatomite-exoskeleton remains of diatomaceous earth or Celite) and a precisely aligned magnet. A magnetic detector is present within the test well. When 2 mL of fresh whole blood is added to the ACT test tube and inserted into the well, Celite activates the intrinsic pathway of coagulation while the magnetic detector senses the magnet within the test tube as the tube rotates slowly. With clot formation, the magnet is lifted up within the tube. As the magnet is displaced, it is no longer sensed by the instrument's magnetic detector. The instrument gives an audible beep and displays the coagulation time.

In this case probably a situation like stuck magnet developed before formation of clot due to presence of a second magnet and the displaced magnet was not sensed by magnetic detector giving rise to falsely low ACT value. This kind of mishaps can be avoided if the ACT tube is regularly inspected for any manufacture defects before adding blood, by looking for clot manually after the machine gives an ACT value and also by using a dual cartridge technique where the second cartridge acts as a control for the first. In addition, one has to be more careful in repeating the dose of heparin in cyanotic babies, where coagulation is already deranged.

   References Top

1.Hattersley PG. Activated coagulation time of whole blood. JAMA 1966;196:436-40.  Back to cited text no. 1
2.Young JA, Kisker CT, Doty DB. Adequate anticoagulation during cardiopulmonary bypass determined by activated clotting time and the appearance of fibrin monomer. Ann Thorac Surg 1978;26:231-40.  Back to cited text no. 2
3.Van Cott EM. Point-of-care testing in coagulation. Clin Lab Med 2009;29:543-53.  Back to cited text no. 3

Correspondence Address:
Bhupesh Kumar
Department of Anaesthesia and Intensive Care, Post Graduate Institute of Medical Education and Research, Chandigarh - 160 012
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0971-9784.109779

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1 Heparin overdose
Reactions Weekly. 2013; 1474(1): 23
[Pubmed] | [DOI]