Assessing acute parathyroid responsiveness in hemodialysis patients by measuring intact parathyrin in pre- and post-dialysis serum.

We measured pre- and post-dialysis concentrations of ionized calcium (iCa) in whole blood, total calcium (tCa) in plasma, and intact parathyrin (PTH) in serum of 19 patients undergoing maintenance hemodialysis. Plasma tCa was inappropriately increased relative to iCa in 63% of the specimens; the iCa correlated with the PTH concentration in 12 of 19 pre-dialysis specimens, whereas tCa correlated with PTH in only five patients. During dialysis, 16 patients had analytically significant changes in iCa (i.e., exceeded the analytical imprecision of 0.04 mmol/L). Pre- and post-dialysis concentrations of PTH were normal in six patients, four of whom showed a detectable response to changes in iCa. Ten patients had increased PTH in at least one specimen; of these, eight had responsive parathyroid glands. Five of the 16 patients had an increased set point for calcium. The minimal PTH responses of two patients suggested refractory hyperparathyroidism. We conclude that routine estimation of iCa, rather than tCa, in dialysis patients markedly improves the identification of patients at risk for secondary hyperparathyroidism, and that measurement of intact PTH in pre- and post-dialysis serum offers a simple means of assessing parathyroid responsiveness in dialysis patients.

Immunoturbidimetric measurement of transferrin.

This paper describes the development of an automated immunoturbidimetric assay for transferrin on a centrifugal analyser. Regression analysis of transferrin values measured immunoturbidimetrically demonstrates good agreement with data obtained by radial immunodiffusion (y = 0.997 + 0.024 g/l, r = 0.980, n = 50). The assay has a detection limit of 1.0 g/l and working range of approximately 1.0 to 6.0 g/l of transferrin. Day-to-day coefficient of variation is less than 3.5%. Immunoturbidimetric transferrin (g/l) and total iron binding capacity values (mumol/l) were compared using an established total iron binding capacity method (y = 0.050 X - 0.030 g/l, r = 0.967, n = 50). Minimal interference was found for lipaemic, haemolysed or icteric samples. Transferrin reference values with a mean of 3.05 g/l and 95% limits from 2.45 to 3.65 g/l were derived using serum from 300 apparently healthy subjects (150 males, 150 females). We conclude that the proposed transferrin method is more reliable and easier to perform than presently available total iron binding capacity methods.

Direct ultraviolet method for enzymatic determination of uric acid, with equilibrium and kinetic data-processing options.

We developed and evaluated a direct ultraviolet method for the enzymatic determination of uric acid in serum, plasma, or urine, without deproteinization of sera and plasma. Equilibrium and nonlinear curve-fitting kinetic options are evaluated and compared, and results of the proposed method are compared with those of a candidate Reference Method. All data-processing options yield a linear relation for absorbance and concentration of uric acid between 0.1 and 2 mmol/L; with the equilibrium option, results are linear to 5 mmol/L. For 100 plasma samples, results correlate well between the proposed method (y) and a reference method (x): y = 0.99x - 0.002 mmol/L. Between-run imprecision is about 2.3%, and interference by hemolyzed, icteric, or lipemic specimens or specimens containing high concentrations of xanthine or paraproteins is minimal. The kinetic option with a data-processing range of 100 s or longer yields results that correlate well with the equilibrium method: y = 1.006x + 0.002 mmol/L (n = 118). For 20 urine samples processed by the proposed (y) and a reference (x) methods, y = 1.04x + 0.038 mmol/L.

Six methods for determining urinary protein compared.

Inter-laboratory surveys have shown that "routine methods" of urinary protein determination are often unsatisfactory. Therefore, we compared six frequently used methods for determination of protein in urine with respect to linearity, within-batch and between-batch precision, comparative bias, and practicability. We assayed dilutions of human and bovine albumin and serum, and fresh and lyophilized human urine. We find that the AACC Selected Method has poor practicability and poor precision under routine conditions, but good linearity. The sulfosalicylic acid/biuret technique is also impracticable, requires a large sample, and is not linear at low concentrations of urine protein. The Coomassie Brilliant Blue technique has a narrow range of linearity and poor precision. The sulfosalicylic acid/sodium sulfate turbidimetric method is not precise and cannot be standardized with bovine materials. The Ponceau-S technique has good performance characteristics and practicability, and we recommend it for routine laboratory use.