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Editor,—I read with interest the article published in the BJO by Hunt.1 In this paper the author reports that the prevalence of APC resistance is more common than any other known thrombophilic state and was present in over one third of patients younger than 45 years with central retinal vein occlusion (CRVO). Therefore, many would argue that a full thrombophilia screening should be performed in a young patient (<50 years old) after CRVO. This screening should be made using an easy to perform technique and providing good discrimination between normal and APC resistant subjects. In this regard, Hunt comments that the biological technique proposed by Dhälback et al 2yields sensitivity and specificity ranging from 85% to 90%, but that it is not reliable if the patient has abnormal clotting such as a lupus anticoagulant or is receiving anticoagulants. For these reasons this technique is not suitable for the screening of patients with venous thrombotic disorders, including CRVO. With regard to this point, it is worth taking into account that our group has proposed a technique,3 which modifies the initial technique proposed by Dhälback et al,2 diluting the sample of the patient’s plasma with plasma lacking factor V. With this modification, both the specificity and sensitivity of the technique is 100%4 and, moreover, it may be used in patients receiving anticoagulant therapy and in those who have lupus anticoagulant.
Thus, in opinion of Dhälback,5 this technique may be suitable for the screening of patients with venous thrombotic disorders since it is very sensitive and specific as well as inexpensive and easy to perform.
Editor,—Since the writing of my editorial on activated protein C resistance and retinal vein occlusion, Dr Aznar has published his modification of the laboratory technique using factor V deficient plasma. It has been taken up widely by coagulation laboratories in the UK in view of its better specificity and sensitivity. When ophthalmologists are screening for activated protein C resistance they should be aware there are now two assays available and ask which assay the laboratory is routinely using.