This study identified 8 out of 14 routine blood factors that were cross-sectionally associated with frailty in older adults based on a nation-wide ageing cohort in the UK. These identified frailty-related blood factors were involved in areas of inflammation, nutrition and metabolism, and demonstrated good performance in frailty detection at a cross-sectional level. Survival analysis further showed that older adults with two or more abnormal blood factors were more likely to develop frailty over a 10-year period compared to their peers without abnormal blood factors.
Our study showed that some inflammatory factors (i.e., hsCRP and WBC) were highly correlated with frailty. Extensive studies have reported the close association between inflammation and frailty. Douglas et.al. have pointed out that inflammation is significantly associated with frailty in men without cardiovascular disease (CVD)30. A meta-analysis reported that frail population have higher levels of CRP, WBC and fibrinogen compared to people who were robust in cross-sectional studies31. As hsCRP and WBC are common indicators of inflammation32, our results have further confirmed the critical role of inflammation in frailty. Studies have found that higher levels of inflammatory markers are associated with greater losses of muscle strength and mass, resulting in reduced physical activity in older adults33,34 and consequently leading to frailty.
Our results identified decreased blood ferritin, Hgb and MCH as risk factors for frailty. Hgb and MCH are common indices for anaemia which has a prevalence from 21–38% among older adults in different populations35,36. The presence of anaemia among older adults has been associated with increased risk of frailty in cohorts37. Moreover, as ferritin is a blood protein for iron storage, low ferritin level suggests an iron deficiency in the body, which can finally cause iron-deficiency anaemia. In respect of physical function, iron deficiency has also been linked to muscle strength and function declines in hospitalized older patients38. When providing older adults with iron supplementation for six months, their grip strength and walking speed were largely improved39. Additionally, iron status can also connect with frailty via its association with inflammatory markers (e.g., CRP40 and IL-641). Since iron, Hgb and MCH deficiencies are indicative of malnutrition, our findings implied the importance of healthy diet, particularly sufficient iron intake, in combating frailty.
The identification of TG and HDL as frailty-related factors indicates the role of lipid metabolism in frailty. Abnormal lipid concentrations can promote the formation of atherosclerotic plaques which affects physical performance with restricted blood supply in muscles and finally leads to frailty16. Beside lipid metabolism, we also found that DHEA was associated with the development of frailty. As a precursor of sex hormone, DHEA can affect skeletal muscle mass and strength via its roles in muscle protein synthesis and lipid metabolism42. Decreased DHEA level has been related to frailty in older adults13 while DHEA therapy was reported to be helpful in improving physical performance43.
Although many studies have examined frailty-related blood factors, few have reported the performance of these factors in frailty screening. Mitnitski et al.44 utilized 40 biomarkers (FI-B) to assess frailty and achieved an AUC of 0.66. Besides the large number of biomarkers involved in Mitnitski’s model, some of the parameters were also not accessible by routine tests, impeding the model from practical use. Based on 21 routine blood factors and blood pressure, Howlett et al.45 created an index of FI-lab for frailty detection and achieved an AUC of 0.72. In our study, we only included 8 routine blood factors and obtained an AUC of 0.758. Moreover, the number of abnormal blood factors was also longitudinally associated with the occurrence of frailty. Therefore, a model with blood factors identified in our study would be convenient for clinical practice in frailty monitoring.
A rising number of studies have reported sex differences in frailty prevalence with women being more likely to become frail than men46. Similar prevalence was also found in our study. Additionally, we further identified different frailty-related blood factors in male and female groups, indicating sex-specific pathways in the development of frailty. The underlying mechanisms of sex differences in frailty are poorly understood. Hormone variations47, epigenetic changes48 and mitochondrial dysfunction49 are suggested to be possible contributors. For instance, frailty is related to inhibited myogenesis while such process is associated with increased myostatin level in men and decreased IGF-1 level in women14. Of note, our findings showed that the model between male and female were totally different, indicating that frailty screening by sex-specific blood factors could be useful based on different purposes.
The large sample size from a nation-wide study is one of the strengths in this study. The sex-specific analysis is also a novel attempt for creating a more robust frailty detection system. Meanwhile, we should admit that due to data availability, this study failed to include some inflammatory biomarkers, such as IL-610, tumour necrosis factor α (TNF-α)50 and tumour necrosis factor β (TNF-β)50, which are commonly reported in frailty association studies. However, we still found that inflammatory factors (i.e., WBC and hsCRP) were closely associated with frailty. Moreover, our findings were based on a UK cohort and one should be cautious when generalizing the results to other populations.