Urinary retinol binding protein: stability and pre-analytical handling of specimens for its measurement

The stability of retinol binding protein (RBP) in urine has been reassessed and was found to be pH-dependent, diminishing as urinary pH decreased. When urines were incubated at 4 degrees C for 1 week, statistically significant losses of RBP were observed at pH values less than seven, varying in degree in different urines. At 37 degrees C some urine specimens showed minor losses when incubated for periods up to 4 h. This was more marked at pH 5 than pH 7 with maximum losses of 26% and 12%, respectively. When adjusted to pH 7, urines showed no significant loss of RBP when stored at 4 degrees C for up to 2 weeks; at -20 degrees C no significant change in RBP concentration was found for at least 14 weeks. To minimize losses of RBP it is recommended that urine pH is adjusted to above seven immediately after voiding.