A42(W)
BIOLOGICAL VARIATION OF BONE-SPECIFIC ALKALINE PHOSPHATASE
AMONGST STABLE HAEMODIALYSIS PATIENTS
Lamb, E1, Sardiwal, S1, Stevens, P2, Delaney M2.
1
Clinical Biochemistry, 2The Kent Kidney Care Centre, East Kent Hospitals University
NHS Foundation Trust, Canterbury
Abnormalities in bone mineral metabolism and vascular calcification are prevalent in patients
with established renal failure on dialysis. Clinical management focuses on biochemical targets,
in particular parathyroid hormone (PTH). However, data has shown that PTH is an imperfect
marker with high biological variation emphasising the need for better markers. A possible
alternative is bone-specific alkaline phosphatase (bALP). The aims of this study were to
evaluate the components of biological variation of total ALP (ALP) and bALP in patients
undergoing dialysis and compare the results to PTH.
ALP and bALP were measured in duplicate in non-fasting serum samples taken twice a week
over a six-week period in 22 stable haemodialysis patients (mean age 68 12y, females:males
1:1) and 12 healthy volunteers (mean age 39 13y, females:males 2:1). bALP was measured
using an immunoenzymetric assay (Ostase®BAP EIA, IDS Ltd). Within-individual (CVI) and
between-subject (CVG) components of variation were estimated on log-transformed data using
ANOVA and used to derive the reference change value (RCV) required to be 95% certain that a
change is significant.
Both CVI for ALP and bALP were significantly higher in the dialysis patients (ALP 9%, bALP
12%) compared to healthy volunteers (ALP 6%, bALP 7%) (p<0.05). Six samples are required
to estimate the homeostatic set point of bALP in a dialysis patient (within +10%) with 95%
probability. In addition, the concentration of serial bALP measurements would need to change
by 34% before this is considered significant (p<0.05). However, variation of bALP (CVI =12%;
RCV=34%, number of samples=6) is significantly lower than that reported for PTH (CVI =26%;
RCV=56%, n = 15) in the same group of dialysis patients (CJASN 2010; 5:1261-1267).
Biological variation of bALP is much lower than that reported for PTH, providing further
evidence for the use of bALP as an alternative marker of bone mineral metabolism in this
setting.