Age dependence of within-subject biological variation of nine common clinical chemistry analytes

The knowledge of biological variation (BV) data is important for clinical decisions and as a basis for defining analytical quality specifications. However, in generating reliable data of biological variation there are still some unsolved problems, such as age dependence. The aim of our work is to verify this aspect.

Twenty-six subjects divided into three groups by age were studied. Blood samples were collected in lithium heparin tubes for four weeks at one week intervals, on the same day of the week (Tuesday) and at the same time of day (8-9 a.m.) by the same phlebotomist. They were analysed in duplicate for creatinine, urate, calcium, albumin, total cholesterol, high density lipoprotein (HDL) and low density lipoprotein (LDL)-cholesterol, triglycerides and iron. After outlier exclusion by Cochran's test, components of biological variation were calculated by ANOVA. The significance of the differences between results of the classes was also calculated with the Student's test (t-test) and the Fisher's test (F-test).

Excluding albumin, the group 3 results (age range from 78 to 98 years) showed significantly lower CV within subjects (CV(W)) than the other two groups.

Our data seem to highlight the relevance of the age when choosing the reference subjects for biological variation studies. The level of within-subject biological variation of the elderly group may have been further reduced by the homogeneity of the group constituted by individuals living together in the same nursing home.