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Aging is a universal phenomenon that affects all normal cells, tissues, organ systems, and organisms. Accordingly, the marrow undergoes changes with age. Age-related hematologic changes are reflected by a decline in marrow cellularity, an increased risk of clonal myeloid neoplams125 and anemia,17,126,127,128,129,130 and a decline in adaptive immunity.131,132,133,134
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MARROW: ANATOMIC CHANGES
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The percentage of marrow space occupied by the hematopoietic tissue declines from 90 percent at birth to a level of approximately 50 percent at age 30 years and 30 percent at age 70 years.135,136 A similar change occurs in the thymus, where involution begins at an earlier age and is reflected anatomically by a reduction in lymphoid mass with an increase in fat137 and functionally by a steady decrease in the production of naïve T cells.80,138 Fat infiltration into the marrow and thymus results in a diminished volume of hematopoietic tissue.
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Although age-related change in the marrow is well described, the exact mechanisms that regulate these changes remains speculative.139 For example, it remains unclear whether the age-associated expansion of marrow fat is a cause or an effect of aging and whether the changes seen in marrow and histologically similar changes within the thymus are intrinsically related. Because of the intricate association of hematologic and immune functions and these common histologic patterns of change with age, both changes in blood and innate immunity are discussed below in the sections on Blood Cell Changes with Age and Aging and Immunity.
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The ontogeny of hematopoietic stem cells is the focus of much attention.140,141 In fetal development the manufacture of blood cells occurs in several organs, but after birth this function is subsumed by the marrow.142,143,144,145,146,147,148 The process of embryonic and fetal hematopoiesis is described in Chap. 7. Hematopoietic cells appear in the medullary cavities of bone around 14 weeks of gestation,149 and by birth the marrow is the primary site of hematopoiesis.
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Unlike the commonly held notion that stem cell compartments diminish either in number or function with age ultimately resulting in an inability to meet homeostatic demands, age-related hematopoietic stem cell (HSC) changes appear to be an exception, at least for murine species in which this question has been most directly addressed.150,151 Early work demonstrated that marrow serially transplanted could reconstitute hematopoietic function for an estimated 15 to 20 life spans.152 Furthermore, the capacity for old marrow to reconstitute proved superior to that of young.153 Subsequently, a number of investigators using a variety of techniques have concluded that HSC frequency in old mice and humans is approximately 2 to 10 times greater than in the young.16,150,151,154,155,156 Some evidence suggests that the intrinsic function of HSCs changes somewhat with age, most notably in a shift in lineage potential from lymphoid to myeloid development. This may contribute to an observed relative increase in neutrophils and decrease in lymphocytes in the blood of older people.157
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There is an intrinsic change in HSCs with age, most notably resulting in a shift in lineage potential from lymphoid to myeloid development. This may contribute to a relative increase in neutrophils and decrease in lymphocytes in the blood of older persons.157 As HSCs age, they accumulate genotypic (mutational) and phenotypic alterations. Indeed, human stem-progenitor cells from healthy volunteers were found to accumulate 13 exonic (private) mutations per year of age.157a Current opinion is that such changes are responsible for the development of immune senescence and that such changes are responsible for the development of immune senescence, as well as the increased occurrence of age-associated diseases such as myelodysplasia and leukemia. Thus, the process of “immunosenescence,” as it affects the innate and adaptive immune system, may result from HSC aging. For example, an age-related decrease in the provision of B-cell precursors may be the result of HSC aging.148
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MARROW DURING ADULT LIFE
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The most apparent change seen in the marrow with aging is decreased cellularity (Fig. 9–1).135 Under normal circumstances, the marrow is the only site of hematopoiesis. Foci of extramedullary hematopoiesis may occur in the liver, spleen, or lymph nodes in pathologic states, but they are not of functional consequence. Until puberty the entire skeleton remains hematopoietically active, but by age 18 years only the vertebrae, ribs, sternum, skull, pelvis, proximal epiphyseal regions of humerus and femur remain active sites of blood production, with other medullary sites infiltrated with fatty tissue. By age 40 years, the marrow in sternum, ribs, pelvis and vertebrae is composed of equal amounts of hematopoietic tissue and fat and cellularity declines gradually thereafter. By age 65 years, marrow cellularity is estimated to be approximately 30 percent,135,136 with a corresponding increase in marrow fat. Age-associated imbalanced bone remodeling and osteoporosis results in decreased trabecular bone which itself may contribute to diminished hematopoiesis.158 The presence of fat correlates with the occurrence and severity of osteoporosis, both of which are evident with aging.159 Several age-related qualitative changes have been identified in hematopoietic cells, including skewed X-chromosome inactivation, telomere shortening,160,161,162 accumulation of mitochondrial DNA mutations,163,164 and micronuclei formation,165 any of which could result in cellular dysfunction. Furthermore, growth hormone production declines with age, and this, too, is linked with deposition of fat within the marrow.166 Administration of growth hormone to old rats reduces marrow fat and increases hematopoietic tissue.167
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BLOOD CELL CHANGES WITH AGE
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Anemia is a significant health problem in the elderly because of a high prevalence and significant associated morbidity, including reduced quality of life, clinical depression, falls, functional impairment, slower walking speed, reduced grip strength, loss of mobility, worsening comorbidities, and mortality.168,169
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In older men and women, anemia defined using the World Health Organization (WHO) criteria of hemoglobin levels less than 13 g/dL for men and 12 g/dL for women170 is associated with an increase in mortality.171,172,173,174,175,176 It has been pointed out that the WHO criteria do not take into account inherent ethnic variations, particularly with respect to Americans of African descent who have lower levels of hemoglobin without significant adverse outcomes.177,178 In a study that analyzed 1018 Americans of African descent and 1583 Americans of European descent adults aged 71 to 82 years, anemia defined by the WHO criteria was associated with increased mortality in those of European descent but not those of African descent.177,178 The reasons for these ethnic differences are undefined. However, the difference is one of degree. In general, the impact of anemia on functional status and mortality in Americans of African descent becomes apparent at hemoglobin levels approximately 1 g/dL lower than in whites. The issue of establishing criteria for the diagnosis of anemia is relevant in the context of age, as well. Older women, for example, have better physical performance and function at hemoglobin values between 13 and 15 g/dL than at between 12.0 and 12.9 g/dL,179 suggesting perhaps that the cut off level of 12 g/dL is too low. Nevertheless, the WHO definition remains the standard used in most current epidemiologic surveys and many clinical laboratories.
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In the third National Health and Nutrition Examination Survey (NHANES III) database, a nationally representative sample of community-dwelling persons and determined age- and sex-specific prevalence rates of anemia in the total U.S. population,127 of those individuals older than age 65 years approximately 11 percent were anemic by WHO criteria (see Table 9–1). The prevalence of anemia was lowest (1.5%) among males between 17 and 49 years of age and highest (26.1%) in males older than 85 years. Among those 65 years and older, the prevalence rate was notably higher in Americans of African descent as compared to Americans of European descent and Americans of Hispanic descent. Prevalence rates of anemia in the elderly vary in community-dwelling and institutionalized populations. Also, anemia is more common among frail elderly. In the nursing home, for example, anemia prevalence approaches 50 percent or higher.126,180,181,182
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Hematologists are usually successful in uncovering the cause of anemia in young adults. However, in older populations a specific explanation cannot be defined by routine evaluation in approximately one-third of anemic patients (Table 9–2).20,183,184 Typically, this anemia is mild (hemoglobin concentration in the 10–12 g/dL range), normocytic, and hypoproliferative (low reticulocyte index). It has been postulated that the cause relates to a number of factors including declining testosterone level,185 occult inflammation,186 impaired renal function with inappropriately low serum erythropoietin,187 or incipient myelodysplasia.188 Likely, unexplained anemia represents an amalgam of these and perhaps other factors, such as shortened red cell survival, refractoriness of the erythroid precursors to erythropoietin stimulation, and/or the presence of as yet undiagnosed illness.
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Serum Erythropoietin and Aging
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Data on erythropoietin levels in nonanemic older persons are inconsistent. Some suggest that nonanemic older persons have higher erythropoietin levels compared to younger adults,189,190,191 but other studies fail to confirm these findings.10,11,12 One longitudinal analysis clearly demonstrates that serum erythropoietin levels rose gradually in healthy individuals who maintained normal hemoglobin levels but the rise was not observed in those who were to develop diabetes or hypertension during the evaluation period.192 An explanation for the rise in serum erythropoietin with age is not established, but in theory, it could be the result of age-associated shortened red cell survival or reduced sensitivity of erythroid progenitor cells to the erythropoietin signal. Studies in older subjects are ongoing to define the basis for the increasing need for erythropoietin to maintain normal levels of red cells.
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Although no significant change is seen in the blood leukocyte count with normal aging,193,194 among those who acquire features of frailty, an increased neutrophil count may be observed.157,195 Furthermore, several qualitative neutrophil defects have been described. For example, a decreased respiratory burst response to soluble signals,193 defective phagocytosis,194 and impaired neutrophil migration to sites of stress196 have been described in accordance with advanced age. Although the exact cause for these functional changes has not been clarified, it may be associated with an age-related alteration in actin cytoskeleton and receptor expression in leukocytes.197 A mild decrease in the blood lymphocyte count is first noticeable in the fourth decade with a gradually progressive decrease thereafter throughout the remainder of the lifespan.198 Qualitative alterations in T-lymphocyte function in the elderly have also been demonstrated.199
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At present, knowledge about the influence of age on platelet counts has been limited to cross-sectional data derived from selected populations. From those data, no or very limited changes in platelet number are noted with age.200,201,202,203 To date, a longitudinal data set describing alterations in platelet number with advancing age has not been produced nor are there conclusive studies describing age-associated changes in platelet function.
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AGING AND COAGULATION
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Coagulation Factors and Aging
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A number of proteins critical to clot formation and fibrinolysis change in characteristic ways with advancing age.204,205,206 Plasma concentrations of factor VII coagulant activity and antigen,204,205,206,207,208 and factor VIIIC,191,206,209 as well as von Willebrand factor,191,209 fibrinogen,191,206,208,210 fibrinopeptide A,191,206,207 and tissue plasminogen activator antigen191,211,212,213 increase with age. In healthy centenarians, levels of activated factor VII, activation peptides of prothrombin, factors IX and X, and thrombin–antithrombin complex concentration were increased, which are signs of higher-than-expected coagulation enzyme activity.206 Age-associated increases in levels of protein C occur in both sexes. Aging is also associated with increasing levels of free protein S.204 In contrast, antithrombin tends to decrease with age in males and increases with age in females following menopause.214 Higher D-dimer and plasmin–antiplasmin complexes indicate an accompanying increase in fibrinolytic activity.206,215 In contrast, plasma tissue-plasminogen activator inhibitor levels increase with increasing age, as do levels of thrombin-activatable fibrinolysis inhibitor in women216 and its proenzyme form, procarboxypeptidase U, in both sexes.217 These latter findings are suggestive of a possible age-dependent compromise in fibrinolytic activity.218 Thus, procoagulant and, in some studies, fibrinolytic activities appear to be increased in older subjects by both in vitro206,219,220 and in vivo studies,190,221 but the changes in fibrinolytic activity are inconsistent. Older patients may show an exaggerated anticoagulant response to warfarin.222
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Aging as a Prothrombotic State
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Activation of the coagulation system and increase in procoagulant markers are associated with the pathogenesis of atherosclerosis.223,224 However, procoagulant markers, most notably D-dimer,225 fibrinogen, and factor VIII,226 also increase with advancing age, and may, in fact, correlate better with aging than with cardiovascular disease.223,224 In a study examining 1729 participants age 70 years and older in the Established Populations for the Epidemiological Study of the Elderly (EPESE) cohort, increasing age was associated with high D-dimer levels. For example, 23 percent of the participants age 90 to 99 years had high D-dimer levels (>600 mcg/L) compared to 13 percent in the 80- to 89-year-old age group and 7 percent in the 70- to 79-year-old age group.215 Investigators measured fibrinogen concentrations in healthy subjects ages 19 to 96 years and found levels to be significantly higher in participants older than age 60 years when compared to younger subjects.227 Healthy individuals across the life span had fibrinogen levels increased by 25 mg/dL per decade of life, and levels as high as 320 mg/dL were found in more than 80 percent of people older than 65 years of age.228 Other markers of activated coagulation, such as plasminogen activating inhibitor-I (PAI-1) and factor VIII also increased with age.209,229,230 Thus, it is now apparent that aging is associated with markers of activated coagulation. In this context, it is notable that the incidence of venous thrombosis and pulmonary emboli increases dramatically in geriatric populations.231,232 Bleeding complications from anticoagulation therapy are also increased in older patients. No interventional study has identified an at-risk population of normal-age subjects without prior thrombosis in whom prophylactic anticoagulation is of value.
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Coagulation and Functional Decline
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In the EPESE study, increases in D-dimer and interleukin (IL)-6 were related to increases in both morbidity and mortality.215 In fact, the correlation for adverse outcomes was stronger with D-dimer than IL-6.233 In this, and other studies,234,235,236 D-dimer and other markers of activated coagulation were associated with limitation in a wide variety of functional domains, including independent activities of daily living (IADL), lower-extremity function, and performance on cognitive testing. The age-associated changes in coagulation markers occur earlier than other aging biomarkers, and hence it has been argued that they could be early predictors of those elderly at increased risk for functional decline.237
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This age-associated prominence of coagulation factors has also been reproduced in animals. For example, when stress associated with physical restraint was compared in aged versus young C57BL/6J mice, significantly increased expression of PAI-1 mRNA was noted in almost all the tissues in older mice.238 Similar results were seen for expression of tissue factor mRNA in aged mice.238 In both these experimental models an increase in microthrombi was noted with clots distributed through multiple organ systems in the older mice.
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In humans, both the presence of depression and/or psychological stress are associated with increased coagulation239,240,241 and decreased fibrinolytic activity.242 In elderly subjects without cardiovascular disease, physical exhaustion, a characteristic frequently used to distinguish frail from nonfrail individuals, was associated with significant increases in both inflammatory and coagulation factors as assessed by fibrinogen, C-reactive protein, and white cell levels.239 Frail and prefrail subjects from the Cardiovascular Health Study had significantly higher levels of fibrinogen, factor VIII, and D-dimer levels as compared to the nonfrail group. The association with frailty persisted even after adjusting for the presence of cardiovascular disease and diabetes.243 Frailty is also associated with increased risk of venous thromboembolism when compared to nonfrail individuals of the same age, especially in association with increased factor VIII levels.244
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Whether related to primary processes of aging or not, the thymus gland undergoes a very characteristic pattern of involution beginning well in advance of other phenotypic changes attributed to aging (see Figs. 9–1 and 9–2; Chap. 6).245 Among the consequences are a decreased generation of naïve T cells.246 Despite this, the total lymphocyte count does not decline greatly because peripheral T cells are capable of expanding to fill the T-cell niche in the absence of generation of new T cells. However, when they do so, the repertoire for antigen recognition becomes less comprehensive. Thymic involution may result from the aging T-cell progenitor population,247 from the defects in rearrangement of T-cell receptor β genes,248,249 from loss of self-peptide expressing thymic epithelium,250 and/or from the loss of thymic trophic cytokines.251 Thymic epithelial cells produce a variety of colony-stimulating factors and hematopoietic cytokines, such as IL-1, IL-3, IL-6, IL-7, transforming growth factor-β, oncostatin M, and leukemia inhibitory factor,252,253,254 which influence the complex process of T-cell production. It is proposed that thymic atrophy and decreased thymopoiesis is an active process and mediated by the upregulation of thymosuppressive cytokines (leukemia inhibitory factor, IL-6, and oncostatin M), which results in the altered peripheral lymphatic tissue T-lymphocyte function with aging.255
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There is a notable shift in the overall blood T-cell population toward lymphocytes with memory T-cell markers,256 and many of these are thought to have attained replicative senescence.257 With the decreasing numbers of naïve T cells in the peripheral lymphatic tissue and increasing memory T cells reaching senescence, elderly persons have difficulties responding to old and new antigens and demonstrate impaired reactions to vaccinations. The decreased efficacy of vaccines may also be a result of alterations in antigen presentation with age. Within the T-helper cell fraction there is a shift to the T-helper (Th) type 2 subset and away from Th1,258 thereby influencing cytokine production and overall immune response.
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In addition to the anatomic changes within marrow and thymus (see Figs. 9–1 and 9–2), similar age-associated morphologic changes within the paracortical and medullary zones of secondary lymphoid tissues (spleen, and lymph nodes) occur, including a decline in the paracortical and medullary zones and increased deposition of fat within the germinal centers.259,260 It remains unclear to what extent these changes contribute to the overall change in immune function with age.
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A wide range of lymphocyte functional changes have been described in the context of aging; however, cataloguing these would be beyond the scope of this chapter. Such changes are detailed in several excellent reviews.24,261,262,263,264 Briefly stated, there is a shift in the T-cell population toward memory T cells,256 which attain replicative senescence in response to repeated antigen exposures.257 With the relative and absolute decrease in numbers of naïve T cells in the peripheral lymphatic tissue and the accumulation of functionally diminished memory senescent T cells, primary and secondary immune responses are reduced in elderly persons.
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Coincident with the age-related changes in lymphocyte function is the increase in levels of circulating proinflammatory cytokines, measureable in some, even in the absence of definable inflammatory disease. IL-6 is the prototype in this regard. In young adults, expression of IL-6 is tightly regulated and serum levels are usually unmeasurable or very low in the absence of inflammatory conditions. Animal studies reveal an increased production of IL-6265,266 from mononuclear cells and lymphoid cells after stimulation with lipopolysaccharide or other mitogens. Similarly, in humans serum IL-6 levels increase significantly with age.267,268,269,270,271 Other inflammatory proteins, including tumor necrosis factor-α (TNF-α) and C-reactive protein are also seen at higher levels in the elderly.272,273,274 Visceral adipose tissue from older mice express greater levels of both IL-6 and TNF-α mRNA than tissue from younger mice,275 and thus, some of the age-associated rise in IL-6 may be the consequence of those metabolic shifts mentioned above.
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CLINICAL CONSEQUENCES
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Although a number of measureable changes occur in the marrow, not the least of which is a reduction in cellularity, apparent compensatory stem cell changes allow the sustenance of normal or near-normal blood counts throughout the life span. It is notable from transplant experience that even when marrow is donated from a 65-year-old person to an human leukocyte antigen (HLA)-matched younger recipient, the donor marrow supports hematopoiesis for the life of the recipient, although allogeneic marrow from older donors has a greater chance of being associated with graft-versus-host disease.276
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To the extent that marrow contains continuously repopulating cell lines, it is quite remarkable that changes attributable to aging alone (i.e., in the absence of disease) are quite subtle. Nonetheless “unexplained anemia” (UA) accounts for up to one-third of cases of anemia in older patients and its frequency increases with advancing age.20
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UA is usually mild, with hemoglobin levels approximately 1 g/dL lower than the WHO standard. The red cells are typically of normal size and examination of the blood film reveals no evidence for intravascular destruction or morphologic features suggestive of myelodysplasia. Although inflammatory cytokine levels may be elevated, the intensity of inflammation is insufficient to produce increased levels of hepcidin; thus, UA has a distinct pathogenesis from the anemia of chronic disease (Chap. 37). Because UA is typically mild, it is likely to be overlooked. In fact, in one population-based cohort that included elderly patients with even more significant anemia, the medical records of affected individuals did not mention anemia as a problem in 75 percent of the cases.171 However, there is now evidence that this casual acceptance of lower hemoglobin levels in older populations may not be advisable.168 Not only can a decline in important functional measures be related to mild anemia,172,277,278,279,280 but longitudinal studies demonstrate increased mortality among individuals with even mild anemia.174,179,281 Furthermore, a retrospective cohort study of the U.S. Veterans Administration National Surgical Quality Improvement database, indicated that of 310,311 subjects age 65 years and older who underwent noncardiac surgery, the 30-day mortality and cardiac event rates increased by 1.6 percent for each 1 percent decrease in hematocrit below the level of 39 percent.275 Thus, although in younger individuals mild anemia may be well tolerated, in many older individuals it is associated with important negative consequences. That stated, it remains to be established whether the correction of anemia for those with UA will result in improved quality of life, physical function, or survival.
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For elderly patients with UA, the presence of macrocytosis; thrombocytopenia; neutropenia; splenomegaly; or unexplained constitutional findings of fever, chills, or weight loss; or symptoms of early satiety; or bone pain should prompt consideration of a marrow examination to rule out myelodysplasia or other diseases affecting marrow function, most of which occur with increasing frequency with advancing age.
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The complex alterations in immune function with age have been described comprehensively in several reviews.261,262,263,264 The changes may explain an age-associated predisposition to certain infections (herpes zoster, tuberculosis reactivation) and perhaps a failure to mount a sufficient vaccine response (e.g., influenza hemagglutinin282,283,284,285,286). The more profound immune deficiency commonly observed in older people most often reflects the debilitating effects of concurrent diseases, most of which occur more commonly with age, and side effects of the medicines used to manage those diseases.
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Inflammation/Coagulation Dysregulation and Frailty
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Presumably on the basis of chronic inflammatory stimuli, there is an age-associated activation of coagulation232 and fibrinolytic287 pathways that favor thrombus formation. Fibrinogen levels are typically high with more than 80 percent of those age 65 years and older having levels above 320 mg/dL.227 Similarly an analysis of D-dimer levels in the EPESE, including 1727 community elderly, revealed an age-associated increase that correlated with declining overall physical function.215 Furthermore, when combining D-dimer and IL-6 levels, those individuals who had elevations of both were at greatest risk for mortality over a 4-year interval.233 In the Cardiovascular Health Study, which included relatively healthy elderly, higher fibrinogen and factor VIII levels were associated with a greater risk for cardiovascular disease and mortality, even after adjustment for other cardiovascular risk factors.232,288 Summarizing what has now become a robust literature, higher IL-6, TNF-α, D-dimer, and C-reactive protein are each associated with negative physiologic consequences, including reduced lower-extremity muscle mass and strength,289,290 cognitive decline,291 insulin resistance,292 subclinical and clinical cardiovascular disease,293,294 renal insufficiency,295 loss of bone mineral density,296 depression,297 anemia,186 dementia,235 and mortality.289 As a result, a general consensus has emerged that activated inflammatory mediators are, at least in part, contributing to the physiology of aging, and to the extent that these pathways are dysregulated, important functional outcomes are impaired.