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High sensitivity C-Reactive Protein (CRP) is one of the 10 variables included in the biological age calculator, PhenoAge (https://michaellustg...-biological-age/). The reference range for CRP is 0 – 3 mg/L, but within that range, what’s optimal? To answer that question, it’s important to know how CRP changes during aging, and what levels are associated with an increased risk of death for all causes.
First, CRP increases 3-5 fold during aging in women and men, respectively (Ferrucci et al. 2005):
Investigating further, CRP continues to increase from relatively low levels of 0.7 mg/L (equivalent to 0.07 mg/dL) in 85-99 year olds to 2.5 mg/L (equivalent to 0.25 mg/dL) in adults older than 110y (Arai et al. 2015):
In support of this finding, the average CRP level in 98 centenarians (average age, 101y) was 5.4 mg/L, when compared with 3.2 mg/L in 70 year olds (Montoliu et al. 2014).
Based on the data for how CRP changes during aging, lower values would be expected to better in terms of risk of death for all causes. How low is optimal for CRP?
Several studies have investigated this issue. Risk of death for all causes was significantly reduced when CRP was < 3 mg/L, when compared with > 3 mg/L in the 11,193 subjects (average age 63y) of Oluleye et al. (2013). In terms of CRP values less than 3 mg/L, CRP < 1.0 mg/L was associated with significantly reduced risk of death for all causes in the 5,248 subjects (average age, 54y) of Hamer et al. (2010), in the 3,620 subjects (average age, 58y) of Koenig et al. (2008), in the 2,240 older adults (average age, 69y) of Elkind et al. (2009), and in the 1,519 subjects (average age 72y) of Kuoppamäki et al. (2015).
Similarly, CRP levels close to 1 mg/dL have also been associated with a significantly reduced risk of death for all causes, including < 0.86 mg/L in the 11,409 adults (average age, 59y) of Shen et al. (2019), and < 0.83 mg/L in the 1,476 men (average age, 53y) of Laaksonen et al. (2014).
In terms of the association between CRP with risk of death for all causes, can we go lower than ~0.8 mg/L? CRP values < 0.5 mg/L were associated with reduced all-cause mortality risk, whereas values > 3 mg/L were associated with increased risk in the 16,850 non-smokers and non-users of hormone replacement therapy (average age, 58y) of Ahmadi-Abhari et al. (2013). Similarly, CRP between 0.5 – 1 mg/L, the area on the chart (see below) where the black line and the shaded 95% confidence interval have a hazard ratio < 1, was associated with a significantly reduced risk of death for all causes, whereas CRP > 5 mg/L was associated with an increased all-cause mortality risk in the 7,015 subjects of Zuo et al. (2016):
Going even lower, CRP < 0.33 mg/L (Tertile 1, white circle) was associated with a maximally reduced all-cause mortality risk when compared with values > 0.86 mg/L (Tertile 3, black circle) in the 1,034 older adults (average age, ~72y) of Shinkai et al. (2008):
Similarly, CRP values between 0.03 – 0.33 mg/L in men and between 0.03 – 0.25 mg/L in women (blue lines for both men and women, Tertile 1) was associated with a significantly reduced risk of death for all causes in the 11,080 subjects (average age, 62y) of Nisa et al. (2016).
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