Challenging Case - Not by Muscle, Race or Ethnicity: Practical use of Cystatin C to Estimate GFR

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* 1. An 80 year-old patient has with multiple morbidities with hyperkalemia and eGFRSCr ≥60 mL/min/1.73 m2. Significant history includes persistent hyperkalemia despite holding ACE inhibitor without other evidence of kidney dysfunction, and poor exercise tolerance in the setting of deconditioning. Personal medical history includes Type 2 diabetes, hypertension, cardiovascular disease, atrial fibrillation, heart failure with preserved ejection fraction, and COPD. Exam is unremarkable except for irregularly irregular heartbeat, and bilateral 2+ edema up to knees. eGFRSCr trend is as follows: 64 → 57 → 64 mL/min per 1.73 m2. A1c is 6.1%.

Which of the following is most correct regarding this patient’s kidney tests?

A. This patient does not have CKD since his eGFR has generally been ≥60 mL/min per 1.73 m2.

B. Cystatin C would help determine whether the patient has CKD

C. The patient has well-controlled diabetes so a microalbumin is not indicated

D. This patient is at low risk of progression because his eGFRSCr is ≥60 mL/min per 1.73 m2

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* 2. What is your level of confidence that your choice is correct?

High

Medium

Low

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* 3. A 60 year-old patient was referred for discrepant eGFRSCr and eGFRCysC. Significant history includes having failure to thrive as described in notes, history of opioid and cocaine use disorder and alcohol use disorder, and chronic hepatitis C infection. The patient is ambulated with a 4-wheel walker and weighs 118 pounds.

His PCP had reported that the patient is generally inactive. More recent labs include eGFRSCr 37 mL/min/1.73 m2 and eGFRCYSC 18 mL/min/1.73 m2.

Which of the following is most correct regarding cystatin C?

A. Cystatin C always reclassifies patients into lower CKD stages.

B. Unlike SCr levels, cystatin C levels are unaffected by medications

C. Unlike SCr, cystatin C is not influenced by muscle mass

D. Cystatin C yields no additional prognostic information beyond SCr

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* 4. What is your level of confidence that your choice is correct?

High

Medium

Low