by W. Todd Penberthy, Contributing Writer
September 24, 2019

ORLANDO — A simple blood-based biomarker already listed in many patients’ medical records was closely associated with risk of hip fracture, a researcher said here.

With data drawn from a large, prospective study of older men, those with relatively high red cell distribution width (RDW) values had nearly triple the risk for hip fracture compared with those with relatively low RDW, reported Kyoung Min Kim with the San Francisco Coordinating Center, who presented the research at the American Society of Bone and Mineral Research annual meeting.

Kim highlighted that “RDW is a simple, universal, and powerful measure to identify those at high risk of hip fracture.”

RDW is a measure of the variability of red blood cell size that is routinely assessed in complete blood counts, and therefore commonly recorded in patients’ charts.

The size of red blood cells becomes more variable with aging in humans. Exceptionally high RDW values have previously been shown to be associated with increased total mortality, cardiovascular disease, renal insufficiency, and dementia, Kim explained, but the strength of association with hip fractures had not been tested yet until this study. Some types of anemia, including iron or B12 deficiency, can also increase RDW.

For the current study, RDW measures were obtained from the Osteoporotic Fractures in Men (MrOS) study which involved 3,635 community-dwelling men with an average age of 79.1 (SD 5.1).

Measures of complete blood cell counts were done with automated CBC analyzers and proximal femur DEXA scans, while fractures were based on reports verified by x-rays, and incident falls were assessed by questionnaires every 4 months, for a total of 12 months.

Follow-up rate was 93.8% for fracture and mortality, with an average follow-up time of 8.1 years over which there were 154 (5%) hip fractures, 701 (19%) total clinical fractures, and 1,059 incident falls observed.

A normal RDW value is considered to be 11%-15%. RDW values were categorized into four intervals: ≤13% (Group 1, n=370), 13.1%-14% (Group 2, n=1,449), 14.1%-15% (Group 3, n=1,120), and >15% (Group 4n=696). Actual observed RDW values ranged from 11.3%-32.9% (median 14.0%; interquartile range 13.5%-14.8%).

Kim and colleagues calculated a hazard ratio of 2.8 (95% CI 1.3-5.9) for hip fracture in Group 4 versus Group 1.

Fracture Risk Assessment Tool (FRAX) score was also calculated. Analysis indicated that combining RDW with bone mineral density (BMD) predicted hip fracture more accurately than FRAX with BMD.

Kaplan-Meier curve analysis revealed that the risk of hip fracture gradually increased with higher RDW values. Hazard ratios for hip fracture relative to Group 1 were 1.7 (95% CI 0.9-3.3) for Group 2, 1.9 (95% CI 1.0-3.8) for Group 3, and 2.3 (1.1-4.7) for patients with RDW >15.1%.

Similarly, risk for falls was significantly increased with RDW.

No association was found between RDW and hip fracture in patients with anemia defined as hemoglobin ≤13.0 g/dL. However, patients with anemia did have an even greater risk for hip fractures independent of RDW values. Furthermore, no association was observed for changes in BMD.

Strengths of the study, said Kim, included that it was a prospective study with validated fracture assessment and standardized data collection on falls, along with a high follow-up rate (exceeding 93%).

Kim concluded that, since RDW and hemoglobin are routinely measured in clinical practice, more and broader research should be performed to test their value in predicting disease risks.

An important limitation of the study was that women were not included.