Age- and sex-related variations in platelet count in Italy: a proposal of reference ranges based on 40987 subjects' data

Abstract

Background and objectives: Although several studies demonstrated that platelet count is higher in women, decreases with age, and is influenced by genetic background, most clinical laboratories still use the reference interval 150-400×10(9) platelets/L for all subjects. The present study was to identify age- and sex-specific reference intervals for platelet count.

Methods: We analysed electronic records of subjects enrolled in three population-based studies that investigated inhabitants of seven Italian areas including six geographic isolates. After exclusion of patients with malignancies, liver diseases, or inherited thrombocytopenias, which could affect platelet count, reference intervals were estimated from 40,987 subjects with the non parametric method computing the 2.5° and 97.5° percentiles.

Results: Platelet count was similar in men and women until the age of 14, but subsequently women had steadily more platelets than men. The number of platelets decreases quickly in childhood, stabilizes in adulthood, and further decreases in oldness. The final result of this phenomenon is that platelet count in old age was reduced by 35% in men and by 25% in women compared with early infancy. Based on these findings, we estimated reference intervals for platelet count ×10(9)/L in children (176-452), adult men (141-362), adult women (156-405), old men (122-350) and, old women (140-379). Moreover, we calculated an "extended" reference interval that takes into account the differences in platelet count observed in different geographic areas.

Conclusions: The age-, sex-, and origin-related variability of platelet count is very wide, and the patient-adapted reference intervals we propose change the thresholds for diagnosing both thrombocytopenia and thrombocytosis in Italy.

Figure 1. Platelet count densities by population.

Each line represents the platelet count distribution of a population; Ogliastra villages have been clustered in three groups following their genotype and phenotype characteristics. Vertical lines represent the reference intervals currently in use.

Figure 2. Platelet count by age in the examined populations.

Figure 3. Platelet count by age and population.

Lines represent age trend of platelet count in the investigated populations.

Figure 4. Mean platelet count in the nine sub-populations along with 95% CI.

Estimates have been obtained by ANOVA adjusting for age and sex. Three strata may be obtained grouping sub-populations according to their mean platelet counts, as evidenced by black circles: Low (Carlantino and North Ogliastra), Medium (Borbera valley, FVG, Others Ogliastra, Cilento and, Molise) and, High (South Tyrol and West Ogliastra).

Figure 5. Platelet counts' reference intervals for clinical practice.

Numbers inside bars represent reference intervals estimated on the overall sample; numbers outside bars represent extended reference intervals estimated stratifying by geographical area.