From the Guidelines
Laboratory tests such as creatinine, glomerular filtration rate (GFR), B-type natriuretic peptide (BNP), troponin, and certain drug levels do not correlate with body mass in patients with obesity, and their interpretation requires careful consideration of obesity-specific factors. Besides D-dimer, several other laboratory tests are affected by obesity, including:
- Creatinine levels, which may appear falsely normal in obese patients despite reduced kidney function because muscle mass doesn't increase proportionally with total body weight 1
- GFR calculations based on creatinine, which can overestimate kidney function in obese patients 1
- BNP and troponin, which are often diluted in obesity, potentially leading to falsely low values that may mask cardiac conditions 1
- Certain drug levels, such as vancomycin, aminoglycosides, and certain chemotherapeutics, which require careful adjustment in obesity to prevent toxicity while maintaining efficacy 1 When interpreting these tests in obese patients, clinicians should consider using obesity-specific reference ranges or adjustment factors where available, and maintain a higher index of suspicion for conditions that might be masked by obesity-related laboratory alterations. Some of the methods for assessing body composition, such as Dual Energy X-Ray Absorpiometry (DXA) and Bioelectrical Impedance Analysis (BIA), have limitations in obese patients, including variability between devices and manufacturers, and the influence of hydration status on results 1. In clinical practice, it is essential to consider these limitations and use a combination of methods to assess body composition and interpret laboratory results in obese patients. The approach to interpreting laboratory results in obese patients should be longitudinal, taking into account factors such as hydration status, and using obesity-specific reference ranges or adjustment factors where available 1.
From the Research
Laboratory Tests and Body Mass Correlation
- The relationship between laboratory tests and body mass in patients with obesity is complex, and several studies have investigated this topic 2, 3, 4, 5.
- Regarding creatinine clearance, a study found that using lean body weight (LBW) or fat-free weight (FFW) in the Cockcroft-Gault equation provided unbiased estimates of creatinine clearance in morbidly obese patients 2.
- Another study suggested that the Cockcroft-Gault formula and the Modification of Diet in Renal Disease (MDRD) formula performed similarly in estimating glomerular filtration rate (GFR) in different weight groups, but the MDRD formula was less accurate in patients with extreme obesity 3.
- A narrative review discussed the evidence and caveats associated with estimated and measured creatinine clearance and GFR assessments in patients with extreme obesity, highlighting the importance of consistent use of the same equation by providers 3.
- A study comparing the Cockcroft-Gault and MDRD formulas in older hospitalized patients found that the Cockcroft-Gault formula slightly underestimates creatinine clearance, while the MDRD formula strongly overestimates it 5.
Liver Function Tests and Body Mass Correlation
- A cross-sectional study analyzed the utility of liver function tests (LFT) and fatty liver index (FLI) in categorizing metabolic phenotypes in a Mediterranean population, finding that LFT were poorly correlated with metabolic phenotypes, while FLI was a useful marker for characterizing metabolically abnormal phenotypes in individuals who are overweight or obese 6.
- The study suggested that FLI may be a useful marker for identifying individuals with non-alcoholic fatty liver disease (NAFLD) and metabolically abnormal phenotypes, regardless of body mass index (BMI) category 6.
Tests Not Correlating with Body Mass
- Based on the available evidence, the following laboratory tests do not correlate with body mass in patients with obesity: