FET Diagnostic test
What is the FET? How it is performed?
The FET is a nonspecific tool used in individuals suspected of having myophosphorylase deficiency or a block anywhere along the glycogenolytic or glycolytic pathway. This test exploits the abnormal biochemistry that results in the absence of those enzymes. Normal muscle generates lactate from the degradation of glycogen when it exercises. With defects in glycolysis or glycogenolysis, pyruvate is not produced under anaerobic conditions, and thus no lactate can be generated by lactate dehydrogenase. In addition, ammonia, inosine, and hypoxanthine concentrations increase significantly. A protocol for FET follows.
1. A blood sample for analysis of baseline lactate, ammonia, and pyruvate levels is drawn through an indwelling catheter in an antecubital vein (preferably placed without use of a tourniquet)
2. In the past, a sphygmomanometer cuff was placed on the upper arm of the side with the indwelling catheter and inflated to at least 20 mmHg above systolic pressure. However, the blood pressure cuff is not necessary and can lead to forearm rhabdomyolysis and compartment syndrome, especially in patients with McArdle disease.
3. The subject rhythmically squeezes a tennis ball or similar object at maximal intensity for 14 seconds followed by a 1-second reprieve for a total of 60 seconds. After the 60 seconds of exercise, additional venous samples for lactate and ammonia are obtained at 1, 2, 4, 6, and 10 minutes thereafter. Pyruvate can be drawn at 2 and 4 minutes after exercise. As they are drawn, the blood vials should be placed on ice and immediately transported to the laboratory and processed.
In normal individuals, lactate and ammonia increase at least two and half times baseline values in the first two samples after exercise and then gradually decrease toward baseline. The major reason for a false-positive result is insufficient work by the individual while exercising. This should be suspected if both lactate and ammonia fail to rise. If lactate (and pyruvate) does not rise but ammonia does, the patient may have a defect in glycolysis, such as McArdle disease. If pyruvate rises but lactate does not, the patient has LDH-m subunit deficiency. In CPT II deficiency, both lactate and ammonium rise appropriately.