Immunoturbidimetry of transthyretin (prealbumin) in human serum.

of a yellow Fe3tcitrate complex [calibration curve (A = 390 nm) was linear from 52.9 to 696.6 pmol/L, n = 8, r = 1.000; SD = 0.9; F = 263224.4; average relative error 0.3%]. Phosphate, which can interfere, is eliminated by precipitation. The usual procedui-e is as follows: To 4 mL of sample, add 0.1 mL of NH4OH (30%) and 0.9 mL of Mg2 (0.2 mol/L) and filter. Then adjust the pH to 2(0.1 mL of 10 molfL HCI) and add 0.25 mL of Fe3 solution (FeC13 . 6H20, 18 mmol/L in HC1, 1 molIL). Read against an iron blank solution (0.25 mL of iron solution in 4.75 mL of HC1, pH 2). Prepare a urine blank by adding 4.75 mL of HC1 solution (pH 2) to 0.25 mL of urine (freed of phosphate) and read against pH 2 HC1 solution. Subtract the absorbance of the urine blank from he absorbance of the urine sample. Citrate concentration is obtained from a calibration graph [reproducibility (n = 8) was 1.9%; recovery (n = 7) was 96 ± 6%]. Carbohydrates and amino acids do not interfere. Concentrations of isocitric, malic, and glyoxylic acids (10 times larger), oxalic acid (two times) and salicylic acid larger than those ever found in urine do not interfere. Lactic acid (present in urine only after great exertion) in the maximal concentration ever seen (6.66 mmolfL) causes a 9% increase in apparent citrate. We compared this method with the citrate lyase “enzymatic” method (Weishman SG, et al., Chin Chim Acta 1973:46:243-6). Regression analysis (n = 18: seven patients with different lithiasis disorders, six normal people, a rat, and four synthetic urine samples) gave r = 0.977, SD = 1.07, F = 352.9. Values for normal people (936, SD 170 mmol/L) were much higher than those for stone-formers (351, SD 156 mmol/L).