This test measures basal and post-exercise venous lactate and ammonia levels. The blood lactate is measured after the fist has been clenched repeatedly for 1 minute with the arm made ischaemic using a sphygmomanometer. Samples are taken at -10 minutes (pre-exercise) and at 0, 1, 3, 5, 10 and 20 minutes following release of the cuff.
A normal response is characterized by a lactate increase of 3-5 fold over baseline with a peak at 1-3 minutes post-exercise, while ammonia increases 5-10 fold over the same time. An absent or inadequate (less than 1.5-fold) increase in lactate is seen in myophosphorylase, phosphofructokinase and distal glycolytic enzyme deficiencies. A normal lactate response with impaired ammonia production is characteristic of myoadenylate deaminase deficiency.
A non-ischaemic test involving maximal intermittent hand-grip exercise for 1 minute has been shown to have the same diagnostic power for McArdle disease as the ischaemic test and is better tolerated in children.
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