Phosphate excretion index

Fig. 7. Phosphate Excretion Index in subjects after more than 24 hours on low and high phosphorus diets.

C. The measurement of the phosphate excretion index (Nil). Since the phosphate/creatinine clearance ratio is normally directly related in a known manner to plasma phosphate concentration (MIO), the observed Cp/Ccr in any given case can be compared with the normal mean value at the same plasma concentration. The difference is called the Phosphate Excretion Index (PEI) and its normal range is —0.09 to +0.09. The values in hyperparathyroid cases usually lie above the upper normal limit and in hypoparathyroid cases below the lower limit (Nil) (Fig. 10). 

Fig. 12. Phosphate Excretion Index in cases of renal calculus and of hyperparathyroidism on basal diet and on aluminium hydroxide showing reduction in PEI and improved discrimination between the two groups [data supplied by R. Fraser (F9)].

Fig. 13. Plasma calcium and Phosphate Excretion Index in 34 cases of primary hyperparathyroidism (r = 0.65 p <0.01). (Circled case in renal failure and omitted from calculation.) [Including data of Reiss (R4) and Reynolds (R5).]...

The ratio of the phosphate clearance (CJ to the creatinine clearance (Ccr) gives an indication of the proportion of phosphate filtered at the glomerulus which has been reabsorbed by the renal tubules (a process in which parathyroid hormone is involved). In order to allow for fluctuations in the level of serum phosphate, the phosphate excretion index (PEI) was devised and this is given by the formula ... 

Urine should be collected in 6 mol/L HCl, 20 to 30 ml for a 24-hour specimen, to avoid precipitation of phosphate complexes. Simultaneous measurement of phosphate and creatinine in serum and urine with fasting morning spot or 1- to 2-hour timed collections permits calculation of the renal phosphate threshold (TmPO /GFR). The clearance of phosphate divided by creatinine clearance can be plotted on a nomogram, and the TmP04/GFR determined. This index expresses phosphate reabsorption as a function of both serum phosphate concentration and GFR and is more useful than urinary phosphate excretion. 

Free pyridoxal either leaves the cells or is oxidized to 4-pyridoxic acid by aldehyde dehydrogenase (which is present in all tissues) and also by hepatic and renal aldehyde oxidases. 4-Pyridoxic acid is actively secreted by the renal tubules, so measurement of the plasma concentration provides an index of renal function (Coburn et al., 2002). There is some evidence that oxidation to 4-pyridoxic acid increases with increasing age in elderly people, the plasma concentration of pyridoxal phosphate is lower, and that of 4-pyridoxic acid higher, than in younger subjects even when there is no evidence of impaired renal function (Bates et al., 1999b). Small amounts of pyridoxal and pyridox-amine are also excreted in the urine, although much of the active vitamin Be that is filtered in the glomerulus is reabsorbed in the kidney tubules. 

As shown in Table 9.5, there are a number of indices of vitamin Be status available plasma concentrations of the vitamin, urinary excretion of 4-pyridoxic acid, activation of erythrocyte aminotransferases by pyridoxal phosphate added in vitro, and the ability to metabolize test doses of tryptophan and methionine. None is wholly satisfactory and where more than one index has been used in population studies, there is poor agreement between the different methods (Bender, 1989b Bates et al., 1999a). 

Metabolism studies in laboratory animals, including the rat, indicate that saccharin is not metabolized (Byard and Golberg, 1973 Mathews et al., 1973 Mineglshi et al., 1972). Saccharin is readily absorbed from the gastrointestinal tract and is rapidly excreted by the kidneys. Saccharin does not form covalently bound adducts with DNA in the bladder of the rat in an assay that would have detected one mole of saccharin per 10 moles of DNA phosphate (Lutz and Schlatter, 1977). By contrast, aflatoxln B has a covalent binding index in liver of 10,300 (Lutz, 1979) compared to a value for saccharin in the bladder of less than 0.05 (Lutz and Schlatter, 1977). Saccharin also appears to be nonmutagenic (Cranmer, 1980). 

Fasting plasma total vitamin B, or more specifically pyridoxal phosphate, is widely used as an index of vitamin B nutritional status. Urinary excretion of 4-pyridoxic acid is also used, but it reflects recent intake of the vitamin rather than underlying nutritional status. 

The metabolism of methionine, shown in Figure 11.22, includes two pyridoxal phosphate-dependent steps cystathionine synthetase and cystathionase. Cystathionase activity falls markedly in vitamin deficiency, and as a result there is an increase in the urinary excretion of homocysteine and cystathionine, both after a loading dose of methionine and under basal conditions. However, as discussed below, homocysteine metabolism is affected more by folate status than by vitamin status, and, like the tryptophan load test, the methionine load test is probably not reliable as an index of... 

The fasting plasma concentration of either total vitamin Bg or, more specifically, pyridoxal phosphate is widely used as an index of vitamin Bg nutritional status, as is the urinary excretion of 4-pyridoxic acid. The generally accepted criteria of adequacy are shown in Table 1. 

---