From the Guidelines

Elevated Cystatin C (CysC) levels can be caused by factors other than decreased kidney function, including inflammation.

Inflammation: Cystatin C levels may be increased by inflammation, as stated in the 2024 Kidney International guideline 1.
Glucocorticoids: Cystatin C levels may also be increased by glucocorticoids, as mentioned in the 2023 American Journal of Kidney Diseases commentary 1.

Cystatin C is a useful marker for estimating glomerular filtration rate (GFR), especially in patients with conditions that affect serum creatinine levels, such as muscle-wasting diseases or malnutrition.
The 2022 American Journal of Kidney Diseases task force report 1 highlights the need to investigate non-GFR determinants of cystatin C, including its potential role as an inflammatory marker.

In clinical practice, it is essential to consider the potential effects of inflammation and other non-GFR determinants on cystatin C levels when interpreting estimated GFR results, as noted in the 2024 Kidney International guideline 1.
The 2023 American Journal of Kidney Diseases commentary 1 suggests using cystatin C-based equations or combining creatinine and cystatin C-based equations for GFR estimation, especially in patients with conditions that may affect serum creatinine levels.

From the Research

Cystatin C is a marker of renal function that appears to be associated with inflammation 2, 3, 4.
Studies have shown that cystatin C levels are positively correlated with inflammatory markers such as C-reactive protein (CRP), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α) 2, 3, 4.
Cystatin C has been found to play a modulatory role in the regulation of natural immunity, protecting against viruses, bacteria, and parasites, and influencing non-specific and specific immune responses 4.

A study on cystatin C-deficient mice found that they were more sensitive to lipopolysaccharide (LPS)-induced sepsis, with increased caspase-11 gene expression and enhanced processing of pro-inflammatory cytokines IL-1β and IL-18 in bone marrow-derived macrophages (BMDM) upon LPS and ATP stimulation 5.
The study suggests that cystatin C deficiency leads to increased NLRP3 inflammasome activation and impaired autophagy in BMDMs, contributing to the excessive inflammatory response 5.
While the studies do not directly investigate the relationship between bone marrow inflammation and elevated cystatin C levels, they suggest that cystatin C plays a role in regulating inflammation and immune responses, which could be relevant to understanding the relationship between bone marrow inflammation and cystatin C levels 2, 3, 4, 5.