Postscript

Introduction

• The reference range (interval) for any blood test is defined as the set of values within which 95% of the normal population falls.
• Reference intervals for platelet counts are important to determine whether a count is normal or not. If the platelet count is abnormal, this could be a first early indicator of potentially treatable disease. 
• Most laboratories in Western countries:
  • Define the normal range (reference interval) for platelet count as 150-400 x 10⁹/L or 150-450 x 10⁹/L, regardless of differences in age, sex or ethnicity.
  • Adopt standard ranges for classification of thrombocytopenia or thrombocytosis (upper and lower limits of the reference interval of 150-400 or 450 x 10⁹/L). 
• The accuracy of reference limits is important for correctly diagnosing thrombocytopenia and thrombocytosis and identifying the underlying cause.
• When the value of a parameter varies according to specific physiological characteristics of investigated individuals, a single reference range is not sufficient, and personalized ranges are required. This is the case for hemoglobin concentration, whose normal values vary according to age and sex.
• Evidence from the literature has raised doubts about the appropriateness of using a single reference interval for the platelet count; for example, it has been shown that the platelet count:
  • Is higher in women than men after puberty.
  • Progressively decreases with ageing.
  • Varies in populations of different origin.
• Despite this evidence and despite an effort by some authors to promote stratification according to sex, age, or other characteristics, sex- and age-independent reference intervals are still most commonly used in clinical practice.
• Should we consider abandoning a single reference interval for all people in favor of personalized normal ranges for platelet counts in clinical practice?

History

• Methods to enumerate platelets prior to the introduction of automated counters in the middle of the 1900s were unreliable and inaccurate and led to a wide range of reference intervals for the platelet count:

• The development of the Coulter Principle in 1953 revolutionized blood counting and resulted in the development of the electronic instruments currently used in our laboratories. About thirty years ago, these instruments were used to study several thousands of blood samples of unselected donors thus defining the reference interval of platelet count as 150-450 x 10⁹/L or 150-400 x 10⁹/L.

Bottom line

• More recent determinations of the reference interval for platelet counts show a general shift to the left (i.e., lower limit below 150 x 10⁹/L and upper limit below 400-450 x 10⁹/L).
• Platelet count varies according to physiological variables:
  • Age:
    • Metanalyses have shown that the platelet count is reduced by 35% in men and by 25% in women relative to early infancy.
    • According to Balduini: “There is no longer any doubt that age is a major determinant of platelet count in healthy people.”
    • It has been proposed that a lower platelet count in the elderly provides a survival advantage or that it represents reduced hematopoietic stem cell reserve. 
  • Gender:
    • Females have a higher platelet count than men (after age 14).
    • These differences are about 10%, which is much lower than those related to aging. 
    • It has been proposed that the difference is explained by higher prevalence of iron deficiency in women. 
  • Genetic background:
    • Several studies have shown that platelet count is highly heritable. 
    • The heritability of variation in platelet count is substantial with estimates ranging from 54% to more than 80%.¹
    • Platelet counts have been shown to differ between geographic isolates in Italy.² 
    • The effect of ethnicity is much less than that of age. 
• Clinical use of thresholds for thrombocytopenia and thrombocytosis:
  • Thrombocytopenia:
    • International Working Group clinical practice guidelines on ITP:
      • A platelet count less than 100 x 10⁹/L was established as the threshold for diagnosis of ITP; they state:
        • A uniform predefined cutoff, instead of local normal ranges or other thresholds based on frequency distribution, is more convenient for practical use and comparisons across studies.
        • This threshold was preferred to the more commonly used level of less than 150 x 10⁹/L, based upon a prospective cohort of otherwise healthy subjects with a platelet count between 100 and 150 x 10⁹/L, showing that the 10-year probability of developing more severe thrombocytopenia (persistent platelet count below 100 x 10⁹/L) is only 6.9% (95% confidence interval [CI], 4.0%-12.0%).
        • Moreover, in some non-Western populations, platelet count values between 100 and 150 x 10⁹/L are frequently found in apparently healthy people.
        • The new cutoff level will also avoid inclusion of most women with pregnancy-related thrombocytopenia, a well known physiologic phenomenon not requiring specific follow-up in the absence of additional clinical features.
    • Why Does My Patient Have Thrombocytopenia?
      • “Thrombocytopenia, usually defined as a platelet count of less than 150,000/mL [150 x 10⁹/L].”
    • Thrombocytopenia:
      • “Normal platelet values range from 150 to 450 x 10⁹/L. There is some debate as to whether patients with platelet counts in the range 100 x 10⁹/L to 150 x 10⁹/L should be designated as having true versus borderline thrombocytopenia; data suggest that most of these patients remain asymptomatic and maintain their platelet counts in this range, whereas a smaller percentage develop immune thrombocytopenia (ITP) with or without a concomitant autoimmune disease”.
    • Thrombocytopenia: an update:
      • “Thrombocytopenia is a common clinical problem defined as a platelet count less than 150 x 10⁹/L although many feel that a cutoff value of 100 x 10⁹ /L is more appropriate to identify clinically significant thrombocytopenia.”
  • Thrombocytosis:
    • British Committee for Standards in Haematology:
      • Thrombocytosis defined by a platelet count >450 x 10⁹/L.
      • The diagnosis of ET requires a sustained thrombocytosis of >450 x 10⁹/L and the exclusion of reactive causes.
    • The WHO diagnostic criteria for Philadelphia chromosome-negative chronic myeloproliferative neoplasms:
      • Platelet count ≥450 × 10⁹/L is a major criterion for essential thrombocythemia.
    • JAMA Diagnostic Test Interpretation:
      • Normal platelet count 150-450 × 10⁹/L.
    • Thrombocytosis: Diagnostic Evaluation, Thrombotic Risk Stratification, and Risk-Based Management Strategies:
      • “The threshold for clinically significant thrombocytosis is variable from patient to patient, and the exact definition of thrombocytosis also varies in the literature, although a platelet count of ≥450 × 10⁹/L is a generally accepted value.”
    • Why Does My Patient Have Thrombocytosis?
      • “Thrombocytosis is defined as having excess platelets in the blood. The normal platelet count in adults ranges between 150,000 and 450,000/mL, regardless of sex and ethnicity. Based on the normal distribution, this implies that a platelet count exceeding 450,000/mL exists in about 2.5% of the population.”
• Should we adopt personalized reference intervals for platelet counts?
  • Normal ranges for platelet counts are not currently reported by clinical laboratories to be different for men vs. women and for older vs younger people.
  • Age, sex and genetic background modulate platelet count in healthy people. 
  • Most of these data were derived from studies in Italian populations, but similar trends have been noted in other populations, including participants in the Third National Health and Nutrition Examination Survey in the United States.  
  • Use of personalized reference intervals is expected to reduce the number of people (especially elderly men) unduly diagnosed with thrombocytopenia. This would spare them of a wrong diagnosis and potentially costly work up. Conversely, the incidence of thrombocytosis in adult males would increase. 

Cohorts

• The normal platelet count in man. J Clin Path. 1951;4:37-46.
  • “The value of platelet counts would… be greatly enhanced if we had more accurate knowledge of what does in fact constitute a normal count, within what limits this is significantly determined, and what does constitute a significant change when successive counts are performed on one individual.”
  • Platelet counts were obtained in 80 healthy individuals using a hemocytometer.
  • The mean platelet count was 241,000 and the ” normal range”140,000-340,000 [140-340 x 10⁹/L].
  • No significant difference was found between the platelet counts on men and those on women.

• The platelet count and mean platelet volume. Br J Haematol. 1981;48(1):31-7.
  • Platelet counts were measured in 5,000 unselected blood specimens, using the Coulter Counter Model S-Plus.
  • In nearly 95% of normal adults the platelet count varied from 150 to 450 x 10⁹/L and the MPV from 7.0 to 10-5 fl.

• Automated platelet-sizing parameters on a normal population. J Clin Pathol 1987; 87: 365-369.

  • The purpose of this study was “to establish tentative reference ranges for the platelet count, MPV, and PCT in a selected population, while the effects of age and sex variables are taken into consideration.”
  • Platelet counts were obtained in 477 ambulatory patients undergoing preadmission evaluation for elective surgery.
  • Platelet counts were measured using the Coulter Model S-Plus IV® electronic cell counter.

• Long-Term Outcome of Otherwise Healthy Individuals with Incidentally Discovered Borderline Thrombocytopenia. PLoS Med. 2006 Mar;3(3):e24.
  • This study was designed with the “aim of elucidating the natural history of apparently healthy adults who were diagnosed with a platelet count between 100 x 10⁹/L and 150 x 10⁹/L, cases that in this study were defined as having ‘‘borderline thrombocytopenia.’’
  • 217 healthy individuals (107 men and 153 women; median age 52 y, range 15–82 y) with incidentally discovered platelet counts between 100 x 10⁹/L and 150 x 10⁹/L were monitored for 6 months to determine whether their condition persisted:
    • 191 (88%) maintained stable platelet counts.
    • These 191 individuals were included in a long-term follow-up study to gain knowledge of their
      natural history; in 65% of cases followed over a median time of observation of 64 months, the thrombocytopenia resolved spontaneously or persisted with no other disorders becoming apparent.
  • The 10-y probability of developing idiopathic thrombocytopenic purpura (ITP), as defined by platelet counts persistently below 100 x 10⁹/l, was 6.9%.
  • The 10-y probability of developing autoimmune disorders other than ITP was 12.0%.
  • Authors concluded: “Healthy individuals with a sustained platelet count between 100 x 10⁹/L and 150 x 10⁹/L have a 10-y probability of developing autoimmune disorders of 12%. Further investigation is required to establish whether this risk is higher than in the general population and whether an intensive follow-up results in an improvement of prognosis.”
• Age- and sex-related variations in platelet count in Italy: a proposal of reference ranges based on 40987 subjects’ data. PLoS One. 2013;8(1):e54289.
  • Data was analyzed from seven studies carried out in Italy on different populations.
  • Data was pooled together from different population-based studies on 40,987 individuals.
  • Platelet count was determined by a Sysmex Hematology analyzer or Beckman Coulter Hematology analyzers.
  • Platelet count distribution was left-shifted with respect to the current reference interval (150-400 x 10⁹/L) and it was also different among populations in Italy:

• • Overall, women had significantly more platelets than men.
  • Platelet count decreased with age:
    • Reduction from infancy to old age of 35% in men and about 25% in women.
    • The observed age-related trend was common to all investigated populations.

• • Platelet count reference intervals were estimated stratifying by three age classes and by sex for individuals over 14 years of age:

• Personalized reference intervals for platelet count reduce the number of subjects with unexplained thrombocytopenia. Haematologica 2015 Sep;100(9):e338-40.
  • The goal of this study was to verify the impact of the personalized ranges (defined in the previous study) in clinical practice.
  • Platelet counts were obtained in 917 Italian adult patients admitted to the Department of Internal Medicine.
  • Cohort was composed primarily of elderly subjects (mean age 74.2±14.3 years), and included 476 men and 441 women.
  • Application of the new personalized reference intervals for platelet counts instead of the traditional one of 150-450 x 10⁹/L, resulted in relevant differences in the number of patients classified as thrombocytopenic or affected by thrombocytosis.
  • The personalized range reduced the number of subjects with unexpectedly low platelet counts by 44.8%
  • In conclusion, using personalized ranges instead of the traditional one had the main effect of reducing the number of subjects with unexplained thrombocytopenia
  • The new reference intervals, therefore, increased the percentage of thrombocytosis subjects by 34.2%.

• Reference Intervals for Platelet Counts in the Elderly: Results from the Prospective SENIORLAB Study. J Clin Med 2020 Sep 3;9(9):2856.
  • Rationale for this study: “Since conditions associated with thrombocytopenia or thrombocytosis occur relatively frequently in seniors and since reference intervals for seniors are generally not well described, we determined reference intervals for platelet counts in seniors within the framework of the SENIORLAB study.”
  • Platelet counts were obtained in 1203 subjectively healthy individuals who were seen for a baseline visit to obtain a detailed history, anthropometric data, and blood by venipuncture under optimal preanalytical conditions.
  • Participants were 60 years or older who resided in Switzerland.
  • Platelet determinations for direct reference intervals were done on a Sysmex XE 5000.
  • There were no age-related changes of platelet counts in females, whereas in males, the lower limit appeared to decline and the upper limit appeared to remain constant.
  • Females had significantly higher platelet counts than males.
  • Authors state: “In seniors, we demonstrate that females have higher reference intervals than males, and that in males, reference limits change with age… Introducing age- and sex-specific reference intervals led to a more balanced frequency of thrombocytopenia and thrombocytosis between the sexes. Abandoning the use of  sex and age-independent reference intervals, at least for seniors, seems advisable.”

• Age-sex–specific ranges of platelet count and all-cause mortality: prospective findings from the MOLI-SANI study. Blood. 2016 Mar 24;127(12):1614-6.
  • Goal was to assess the predictive values of newly defined ranges of platelet count (Biino et al) for all-cause mortality.
  • Analyses on 21,635 adult individuals 35 years of age or older randomly recruited from the general population of the MOLI-SANI study.
  • Platelet count (x 10⁹/L) intervals used:
    • 156-405 < 65 yo F 
    • 140-379 > 64 yo F 
    • 141-362 < 65 yo M
    • 122-350  >64 yo M
  • During a median follow-up of 7.6 years.
  • Compared with the normal range:
    • Lower platelet number was significantly related to increased risk of mortality in the multivariable model
    • Higher platelet count was not associated with higher risk of death.
  • Using personalized (sex- and age-specific) rather than traditional reference intervals of platelet count reduces the number/proportion of subjects with thrombocytopenia, as lately observed in a sample of Italian adult patients to whom the newly identified cutoffs were applied.
  • The group of possibly true thrombocytopenic subjects (identified by personalized range intervals) had a higher risk of total mortality compared with subjects classified as thrombocytopenic by traditional range intervals.