Fecal collection, problems

Apparent absorption. When apparent absorption, or the amount of zinc fed minus the amount eliminated in the feces, is used to determine bloavallabillty, two problems must be considered. First, zinc collected Includes non-absorbed dietary zinc, endogenous zinc from bile and pancreatic secretions into the small intestine, and sloughed intestinal cells, as shown in Figure 1 ( ). In addition to these sources, intestinal mixing occurs and the dietary zinc consumed on one day is eliminated over many days. Figure 2 ( ) shows the excretion pattern of one group of subjects, in which total excretion of unabsorbed took from 6 to 15 days. The fecal composites... 

Available analytical performance data for fecal fat measurements in the UK also indicate that the test should now be consigned to history. Eighty-two per cent of laboratories use no internal quality control and EQA is impractical. When the titration step was assessed in an EQA exercise, between-laboratory coefficients of variation for three samples ranged from 31% to 42%. Infrared spectroscopy offers the possibility of improved within- and between-laboratory precision for fecal fat measurements, but does not address the problems of dietary input and sample collection, and is unlilcely to be available to most laboratories. 

Studies that have included measurement of urinary and fecal excretion of radioactivity have helped to confirm these studies of total volumes of distribution of the thyroid hormones. When excreted radioactivity is subtracted from injected radioactivity and the result divided by plasma radioactivity (all in the same units and corrected for radioactive decay), the quotient is taken as the volume of distribution. The problem with this approach is that it is difficult to ensure complete collection of excreta. 

---