Validation of a locally derived serum calcium adjustment equation: relationship with total 25(OH) vitamin D and PTH levels.
OBJECTIVE
Many clinical laboratories in the UK use a standard equation to adjust total calcium for albumin concentration. To assess the validity of this practice, we assessed the effect of the use of a traditional and locally derived calcium adjustment equation on parathyroid hormone (PTH) and 25(OH, hydroxy) vitamin D levels.


METHODS
Samples requested for calcium and albumin measurement over a 6 month period that met inclusion criteria were used to derive a calcium adjustment equation (n=60 941). The traditional and locally derived calcium adjustment equations were then applied to a second cohort of adult patients that underwent calcium measurement over a 1 year period (n=275 456). Patients were classified as hypocalcaemic, normocalcaemic or hypercalcaemic using a UK Pathology Harmony adjusted calcium reference interval (2.2-2.6 mmol/L).


RESULTS
The local calcium adjustment equation provided a 7.1-fold reduction in the prevalence of hypocalcaemia. Patients classified as hypocalcaemic using the locally derived equation had statistically significantly lower 25(OH) vitamin D and higher PTH levels. A 2.4-fold increase in the prevalence of hypercalcaemia was also observed using the new equation, but with no significant difference in 25(OH) vitamin D or PTH levels.


CONCLUSIONS
A locally derived calcium adjustment equation reclassified the calcium status of 61 278 (22%) patients. Patients classified as hypocalcaemic by the locally derived equation had significantly lower 25(OH) vitamin D and significantly higher PTH values, providing evidence that use of this adjustment equation generates adjusted calcium results of greater clinical relevance. This study provides further and novel evidence that individual laboratories should determine local equations for adjusted calcium where possible.
Department of Clinical Biochemistry, Pathology at Wigan and Salford, Salford Royal NHS Foundation Trust, Stott Lane, Manchester, UK.
Jonathan Joseph
JJ
Scargill
Department of Clinical Biochemistry, Pathology at Wigan and Salford, Salford Royal NHS Foundation Trust, Stott Lane, Manchester, UK.
John Mark
JM
Guy
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002118:Calcium / Q000097:blood*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010281:Parathyroid Hormone / Q000097:blood*; D012709:Serum Albumin; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D055815:Young Adult
D010281:Parathyroid Hormone; D012709:Serum Albumin; D014807:Vitamin D; C104450:25-hydroxyvitamin D; D002118:Calcium
BIOCHEMISTRY; CALCIUM METABOLISM; CHEMICAL PATHOLOGY; EVIDENCE BASED PATHOLOGY; LABORATORY TESTS