What happened to my Cystatin C test results to confirm my impaired eGFR?

What Likely Happened to Your Cystatin C Test Results

Your cystatin C test is likely delayed because many laboratories in the United States do not routinely perform this assay on-site and must send specimens to reference laboratories, which can take several days to over a week for results to return. 1

Why Cystatin C Testing Takes Longer

Laboratory Availability Issues

• Cystatin C assays are not widely available in many countries, including the U.S. and Canada, despite being recommended by guidelines 2, 3
• Most clinical laboratories must send cystatin C samples to specialized reference laboratories rather than performing the test in-house 1
• This send-out process typically adds 3-7 days (or more) to turnaround time compared to routine creatinine testing

Technical Requirements

• Cystatin C requires specific assay calibration traceable to international standard reference materials, which not all laboratories maintain 2
• The test requires specialized equipment and trained personnel that many routine clinical laboratories lack 4

Why Your Doctor Ordered This Test

With your eGFR of 58 mL/min/1.73 m², you fall into the exact category where KDIGO guidelines suggest measuring cystatin C to confirm whether you truly have chronic kidney disease. 1

The Clinical Rationale

• Guidelines recommend cystatin C for adults with creatinine-based eGFR of 45-59 mL/min/1.73 m² who lack other markers of kidney damage (such as proteinuria) 1, 2
• If your cystatin C-based eGFR comes back ≥60 mL/min/1.73 m², the diagnosis of CKD would not be confirmed 1, 2
• If your cystatin C-based eGFR is also <60 mL/min/1.73 m², then CKD diagnosis is confirmed 1, 2

Important Context About This Testing Strategy

• Research shows that approximately 23% of patients with creatinine-based eGFR in your range (45-59) actually have normal kidney function when confirmed with cystatin C 2
• However, 65% of patients tested have a cystatin C-based eGFR that is lower than their creatinine-based eGFR, meaning more people are reclassified to worse kidney function than better 5

What to Expect When Results Arrive

Interpretation Framework

• Your results will include an eGFR calculated from cystatin C alone (eGFRcys) and likely a combined creatinine-cystatin C equation (eGFRcreat-cys) 1
• The combined equation (eGFRcreat-cys) provides the most accurate GFR estimation and should guide clinical decisions 1, 2

Potential Outcomes

• If eGFRcys ≥60: Your declining creatinine-based eGFR may reflect factors other than true kidney disease (such as muscle mass changes, diet, or medications) rather than actual kidney damage 1, 2
• If eGFRcys <60: This confirms reduced kidney function and warrants appropriate CKD management and monitoring 1, 2

Common Pitfalls to Avoid

• Do not assume the delay means something is wrong with your kidneys—the delay is purely logistical due to laboratory processing 1
• Be aware that cystatin C can be affected by thyroid dysfunction, smoking, and high-dose corticosteroids, which should be considered when interpreting results 2, 6
• If you have significant muscle mass changes, malnutrition, or chronic inflammatory conditions, both creatinine and cystatin C may have limitations 2, 6

Cost Considerations

• Cystatin C testing costs more than routine creatinine testing, though the "triple marker" panel (creatinine, cystatin C, and urine albumin) is considered affordable in high-income settings 2
• The higher cost is one reason why cystatin C has not been widely adopted despite guideline recommendations 2, 7