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Epic Test Code LAB3767 Cystatin C with Estimated Glomerular Filtration Rate (eGFR), Serum

Additional Codes

MML:CSTCE


Specimen Required


Collection Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 0.5 mL

Collection Instructions:

1. Serum gel tubes should be centrifuged within 2 hours of collection.

2. Red-top tubes should be centrifuged and serum aliquoted into a plastic vial within 2 hours of collection.


Useful For

Assessing kidney function in patients suspected of having kidney disease

 

Monitoring treatment response in patients with kidney disease

 

An index of glomerular filtration rate (GFR), especially in patients where serum creatinine may be misleading (eg, very obese, older adults, or malnourished patients)

 

Calculation of Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) cystatin C estimated GFR for patients where serum creatinine may be misleading (eg, very obese, older adults, or malnourished patients)

Method Name

Immunoturbidimetric

Reporting Name

Cystatin C with Estimated GFR, S

Specimen Type

Serum

Specimen Minimum Volume

0.25 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 14 days
  Frozen  30 days
  Ambient  7 days

Reject Due To

Gross hemolysis Reject
Gross lipemia OK
Gross icterus Reject

Clinical Information

Cystatin C is a low-molecular weight (13,250 Da) cysteine proteinase inhibitor that is produced by all nucleated cells and found in body fluids, including serum. Since it is formed at a constant rate and freely filtered by the kidneys, its serum concentration is inversely correlated with the glomerular filtration rate (GFR); ie, a high concentration indicates a low GFR, while a lower concentration indicates a higher GFR, similar to creatinine.

 

The renal handling of cystatin C differs from creatinine. While both are freely filtered by glomeruli, once filtered, cystatin C, unlike creatinine, is reabsorbed and metabolized by proximal renal tubules. Therefore, under normal conditions, cystatin C does not enter the final excreted urine to any significant degree.

 

The serum concentration of cystatin C is not greatly affected by body mass, age, sex, or race. Thus, in certain cases, cystatin C may be a more reliable marker of kidney function (ie, GFR) than creatinine.

 

GFR can be estimated (eGFR) from serum cystatin C utilizing an equation that includes the age and sex of the patient. The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) cystatin C equation was developed by Inker et al(1) and demonstrated good correlation with measured iothalamate clearance in patients with all common causes of kidney disease, including kidney transplant recipients. Cystatin C eGFR may have advantages over creatinine eGFR in certain patient groups whose muscle mass is abnormally high or low (for example quadriplegics, much older adults, or malnourished individuals). Blood levels of cystatin C also equilibrate more quickly than creatinine, and therefore, serum cystatin C may be more accurate than serum creatinine when kidney function is rapidly changing (eg, amongst hospitalized individuals).(2)

 

The same group also developed an eGFR equation that uses serum creatinine and cystatin C, in addition to age, sex, and race.(1) This equation may be useful to average out potential confounders of creatinine versus cystatin C.

Reference Values

CYSTATIN C:

18-49 years: 0.63-1.03 mg/L

≥50 years: 0.67-1.21 mg/L

0-17 years: Reference values have not been established. Refer to estimated glomerular filtration rate (eGFR).

 

ESTIMATED GFR:

>60 mL/min/BSA (body surface area)

Adult eGFR: Estimated GFR calculated using CKD-EPI Cystatin C equation.(1)

Pediatric eGFR: Estimated GFR calculated using Schwartz Cystatin C equation.(12)

Interpretation

Cystatin C:

Cystatin C inversely correlates with the glomerular filtration rate (GFR), that is, elevated levels of cystatin C indicate decreased GFR. Cystatin C may provide more accurate assessment of GFR for very obese, older adults, or malnourished patients than creatinine. Cystatin C equation does not require patient ethnic data and can be used for those patients with this information unavailable.

 

Due to immaturity of kidney function, cystatin C levels are higher in neonates less than 3 months of age.(3)

 

Estimated GFR:

Chronic kidney disease (CKD) is defined as the presence of persistent and usually progressive reduction in GFR (GFR <60 mL/min/1.73 m[2]) and/or albuminuria (>30 mg of urinary albumin per gram of urinary creatinine), regardless of GFR.

 

According to the National Kidney Foundation Kidney Disease Outcome Quality Initiative (KDOQI) classification, among patients with CKD, irrespective of diagnosis, the stage of disease should be assigned based on the level of kidney function.(4)

 

Table. Kidney Disease: Improving Global Outcomes (KDIGO) guidelines provide the following GFR categories

 

Stage

Description

GFR mL/min/BSA

1

Kidney damage with normal or increased GFR

90

2

Kidney damage with mild decrease in GFR

60-89

3A

Mild to moderate decrease in GFR

45-59

3B

Moderate to severe decrease in GFR

30-44

4

Severe decrease in GFR

15-29

5

Kidney failure

<15 (or dialysis)

Cautions

The estimated glomerular filtration rate (eGFR) is not a precise measure of GFR and can be influenced by nonrenal factors such as inflammation.

Day(s) Performed

Monday through Sunday

Report Available

Same day/1 to 3 days

Specimen Retention Time

7 days

Performing Laboratory

Mayo Clinic Laboratories in Rochester

CPT Code Information

82610

NY State Approved

Yes

Forms

If not ordering electronically, complete, print, and send one of the following forms with the specimen:

-General Test Request (T239)

-Cardiovascular Test Request Form (T724)

-Renal Diagnostics Test Request (T830)