Reference interval for phosphohexose isomerase activity in aqueous humor.

A solitary report in the literature suggests that determination of phosphohexose isomerase (PHI; glucosephosphate isomerase, EC 5.3.1.9) activity in aqueous humor and serum may be a useful adjunct in the diagnosis f retinoblastoma (1). The aqueous humor/serum PHI ratio of 20 patients with retinoblastoma was compared with that for eight “controls” having intraocular diseases other than retinoblastoma, but the normal range of PHI activity in aqueous humor was not defined (1). Establishing the normal range is obviously necessary to detect departures from normal in pathological conditions. Collection of aqueous humor is risky and should not be done on completely normal persons. Because senile cataract is a part of the normal ageing process, patients with senile cataract have been taken as “healthy” controls in establishing the normal range of aqueous humor lactate dehydrogenase (EC 1.1.1.27) activity (2). We collected specimens of aqueous humor from 25 patients (14 men and 11 women; age range 50-80 years) with senile cataract who were otherwise healthy. The specimens were collected by limbal puncture (corneal side) at the time of cataract surgery. Venous blood specimens were also collected at the same time. PHI activity was determined in each specimen of aqueous humor and serum (3). The mean (and SD) aqueous humor PHI activity was 7.8 (3.0) Bodansky units (BU), with a range of 2-15 BU. The distribution was nearly normal. Therefore, the normal range may be taken as 1.8-13.8 BU (mean ± 2 SD), which may be rounded off to 2-14 BU. This range actually covers 96% of the individual data in our sample. The PHI activity ratio for aqueous humor/serum ranged between 0.10 and 0.50. The mean (and SD) was 0.32(0.09). The distribution was almost normal. Therefore, the normal range for this ratio may be taken as 0.14-0.50 (mean ± 2 SD), which again covers 96% of our individual data.