[Delayed measurement of PTH in patients with CKD: storage of the primary tube in the dialysis unit, which temperature? Which kind of tube?].

Parathyroid hormone (PTH) is measured in patients with chronic kidney disease (CKD) to evaluate the spread of secondary hyperparathyroidism and to identify renal osteodystrophy subtypes. An important intermethod variability that can significantly influence the clinical decision has been highlighted recently. Similarly, it is acknowledged that the preanalytical conditions are important to optimize the interpretation of a PTH level by comparison with the K/DOQI guidelines. Considering the frequent case of a dialysis patient in whom blood is handled in the evening and addressed to the clinical laboratory the next morning, we have evaluated the stability of the PTH concentration during a 18-hour period. We thus measured PTH with three automated assays in three kinds of tubes (plain tubes with a gel separator, EDTA tubes, EDTA+aprotinin tubes) which were either immediately centrifugated with a prompt freezing of the serum or plasma, or stored for 18hours at room temperature or at 4 degrees C. Our results demonstrate that, whatever the kind of tube, the PTH concentration is not altered by a 18-hour storage at 4 degrees C which is not the case at room temperature. Using a tube with a gel separator necessitates however to centrifugate the tube in the dialysis unit. On the other hand, the use of serum, by contrast with EDTA plasma, allows the measurement of other biological parameters including calcium, does not need that the tube is fully filled and, according to our results, reduces the intermethod variability. In conclusion, this study shows that the measurement of PTH may be delayed by 18hours if the primary tube is kept at 4 degrees C. Assuming that the primary tube is centrifugated but not opened in the dialysis unit, serum may be the sample of choice for the measurement of PTH in patients with CKD.