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CKD-EPI and MDRD equations are better predictors of GFR than DTPA scan using Gates protocol

Dr. A. V. Mulay1 DNB, MSc;
Dr. Shefali Gokhale2 DNB.
Departments of Nephrology1 and Nuclear Medicine2,
Inlaks and Budhrani Hospital, Pune.


  • Accurate estimation of glomerular filtration rate (GFR) is an important part of evaluation of kidney function.
  • Renal scan using radiolabels dietnylenetriaminepentaacetic acid (99mTc-DTPA) by Gates protocol is commonly used for estimating GFR.
  • MDRD and Cockroft-Gault equations are routinely used to estimate GFR.
  • CKD-EPI equation was developed recently and was shown to have better performance compared to MDRD equation.1

Objectives

  • To compare performance of serum creatinine based estimating equations and 99mTc-DTPA scintigraphy (Gates protocol) in estimating Glomerular Filtration Rate using plasma clearance of 99mTc-DTPA as ‘Gold Standard’.

Methods
  • Prospective Cohort Study
  • Measured GFR (‘Gold Standard’):
    • GFR was measured by plasma clearance (GFRp) of 99mTc-DTPA using multiple sampling at 120minute, 180minute and 240minute after injection of 99mTc-DTPA.2
    • The counts were plotted on an excel sheet to obtain GFR corrected to body surface area.

Sample plot and results of GFR by plasma clearance of 99mTc-DTPA


Methods (Contd)
  • Estimated GFR:
    • Cockroft Gault equation and 4-variable MDRD equation was used to estimate GFR (GFRCG and GFRMDRD).3
    • CKD-EPI equation expressed on the natural scale was used to estimate GFRCKD-EPI.1
    • We also determined GFR by 99mTc-DTPA renal scan (GFRs) using GE Hawkeye-4 duel headed gamma camera and Gates protocol. (GFRS).
  • We compared GFR measured by gold standard with estimated GFR by other methods using Pearson and Spearman correlation coefficient.
  • We also determined bias and precision of estimated GFR by each method.2,3

Results
  • 29 Patients with age ≥ 18Years of Age
  • Male - 20; Female - 9.
  • Mean Age - 48.5 (15.9) Years.
  • Mean GFR – 75.9 (24.7) ml/min/1.73m2.
  • Correlation of GFR was best with estimated GFR by CKD-EPI equation. (rp = 0.74, rs = 0.75).
  • CKD-EPI equation also had least bias and best precision followed by MDRD equation.
  • 99mTc-DTPA scan with Gates protocol had the worst bias and precision.

Table 1 – Performance of various methods for estimating GFR

Pearson Correlation Coefficient (rs)

Spearman Correlation Coefficient (rs)

Bias

Precision

GFRCKD-EPI

0.74

0.75

0.25

20.8

GFRMDRD

0.65

0.74

0.95

24.4

GFRCG

0.53

0.74

4.1

28.6

GFRS

0.64

0.60

-22.3

40.8


Limitations
  • Relatively small number of patients.
  • Included patients mostly in CKD stage I / II / III.
  • Not validated in children (Age < 18 years).

Conclusion
  • CKD-EPI equation is the best predictor of GFR in adults with stage I – III CKD, followed by MDRD equation.
  • 99mTc-DTPA scan with Gates protocol should not be used to estimate total GFR since it performs poorly compared to serum creatinine based estimating equations.

References
  1. A new equation to estimate Glomerular Filtration Rate. Ann Intern Med. 2009 May 5; 150(9): 604–612.
  2. Estimating glomerular filtration rate in kidney transplantation: A comparisn between serum creatinine and cystatin C based methods. J Am Soc Nephrol 16: 3763–3770, 2005.
  3. Evaluation of laboratory measurements for clinical assessment of kidney disease (K-DOQI guidelines). American Journal of Kidney Diseases, Vol 39, No 2, Suppl 1 (February), 2002: pp S76-S11.0

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