What does IDMS stand for?
Isotope dilution mass spectrometry
(IDMS) reference measurement procedure.
There are several reasons (and implications) why this
new standardized value (creat-IDMS) is being reported by labs
throughout the world, and the conventional value is being
eliminated.....
Here are some direct quotes from a valuable reference site:
https://www.nkdep.nih.gov
1.Following implementation of revised calibration for serum
creatinine methods, use of the IDMS-traceable Modification of
Diet in Renal Disease (MDRD) Study equation will give a more
accurate value for eGFR in adults.
[Dave: wondering why? See "Serum creatinine assay errors"
below.
2.The serum creatinine reference interval will change, in
most cases, to lower values. The magnitude of change is
likely to be between 5 to 20 percent.
[Dave: The greater accuracy of the
IDMS value produces slightly lower creatinine values. When
these lower values are used in conventional equations such as
the Cockcroft and Gault equation, higher than expected
results (clearances) are expected. ]
3.Creatinine clearance values based on measured serum and
urine creatinine results may change. A new reference interval
and interpretive criteria may need to be established for
creatinine clearance.
4.Following implementation of revised calibration for serum
creatinine methods, creatinine clearance estimating equations
such as Cockcroft-Gault, Schwartz, or Counahan-Barratt will,
in most cases, give values that are higher than those values
obtained before creatinine method recalibration.
Pharmaceutical manufacturers have used the Cockcroft-Gault
equation to estimate kidney
function, but there is no version of
this equation available for use with an IDMS-traceable
creatinine value.
Serum Creatinine Assay Errors and Revision
Since serum creatinine is the variable with the greatest
impact in GFR prediction equations, the need for reliable
serum creatinine measurements is of utmost importance.
Commonly used creatinine assay methodologies include (1)
alkaline picrate methods (e.g., Jaffe method [classic] and
compensated [modified] Jaffe methods [used in MDRD study],
(2) enzymatic methods, (3) high-performance liquid
chromatography, (4) isotope dilution mass spectrometry
(IDMS), (5) gas chromatography, and (6) liquid
chromatography. Many factors may produce inherent errors in
these assay methodologies, including patient-specific
variables (diseases, dietary factors, pregnancy, hydration
status, serum glucose values, blood pressure), variations in
GFR measurements (measurement of serum and urine filtration
markers, urine flow-rate assessment and collection), and
errors in creatinine measurements due to the presence of
interfering substances and assay methodology inaccuracies.
With the commonly used classic and modified Jaffe methods, up
to 20% of the color reaction may be due to the presence of
noncreatinine chromagens in the serum sample, thereby
overestimating serum creatinine and therefore underestimating
CLcr. This error is most significant within the normal
reference range of serum creatinine values.
Other commonly used assays, such as the kinetic alkaline
picrate assay, also report a serum creatinine value higher
than actual (positive bias), thereby underestimating CLcr
most significantly within the upper range of normal for
creatinine values—values that define early-onset CKD.[16] A
study of the effects of the Jaffe alkaline picrate assay
procedure found that this method overestimates
MDRD-calculated GFRs by approximately 50% in patients with a
serum creatinine of 1.75 mg/dL.[17]
An NKDEP laboratory working group, formed for the purpose of
reevaluating serum creatinine assay performance worldwide,
determined that various serum creatinine assays are
suboptimal for use as a universal predictor of a patient's
actual serum creatinine value[16,18-22] and recommended
reporting calculated GFR values over 60 mL/min/1.73 m2 as
simply >60 mL/min/1.73 m2 when older serum creatinine assays
are used. The NKDEP working group concluded that the lack of
calibration standardization traceable to a single accurate
standard results in differing degrees of accuracy with serum
creatinine assay methods used by various assay kit
manufacturers. The group further concluded that all current
equations for estimating GFRs, including the MDRD equations
and the Cockcroft–Gault equation, are less accurate in
patients with normal and slightly increased serum creatinine.
[23] This led the group to prepare recommendations to
standardize and improve creatinine measurement.[24]
The NKDEP's Creatinine Standardization Program, in
conjunction with the Food and Drug Administration, is
encouraging all manufacturers of creatinine assay kits to
recalibrate routine serum creatinine methods to be traceable
to an IDMS standard and to work with clinical laboratories to
coordinate the release of this recalibrated assay with the
introduction of a revision of the MDRD4 equation to estimate
the GFR appropriate for use with these new assay standards.
This new equation (MDRD4revised), in conjunction with the
IDMS-traceable assay, allows GFR estimates to be reported up
to 90 mL/min/1.73 m2; that is, only values over 90 should be
reported as >90 mL/min/1.73 m2. The NKDEP working group also
developed a website for health care professionals to provide
and explain their creatinine standardization program
recommendations and has added recommendations specifically
for pharmacists for drug dosing.
Although the working group currently recommends the use of
the revised four-variable MDRD equation for laboratories also
using the IDMS-traceable creatinine assay, it also recommends
the use of the original four-variable MDRD equation in the
interim until all clinical laboratories can begin using the
newer IDMS-traceable serum creatinine assay.
https://www.nkdep.nih.gov/labprofessionals/Pharmacists_and_Authorized_Drug_Prescribers.htm
Original MDRD Study Equation
Conventional units
GFR (mL/min/1.73 m2) = 186 x (Scr)-1.154
x (Age)-0.203 x (0.742 if female) x (1.212 if
African-American)
SI units
GFR (mL/min/1.73 m2) = 186 x (Scr/88.4)-1.154
x (Age)-0.203 x (0.742 if female) x (1.212 if
African-American)
NOTE: This equation should be used
only with those creatinine methods that
have not been calibrated to
be traceable to IDMS.
IDMS-traceable MDRD Study Equation
Conventional units
GFR (mL/min/1.73 m2) = 175 x (Scr)-1.154
x (Age)-0.203 x (0.742 if female) x (1.212 if
African American)
SI units
GFR (mL/min/1.73 m2) = 175 x (Scr/88.4)-1.154
x (Age)-0.203 x (0.742 if female) x (1.212 if
African American)
NOTE: This equation should be used
only with those creatinine methods that
have been calibrated to be
traceable to IDMS. If you do not know whether your laboratory
uses a method that has been calibrated to be traceable to
IDMS, talk to your in vitro diagnostics manufacturer
representative.
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References
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1) Cockcroft DW, Gault MH. Prediction of creatinine
clearance from serum creatinine. Nephron 1976;16(1):31-41
2) Davis GA, Chandler MH. Comparison of creatinine clearance
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Pharm 1996;53:1028-32.
3) Dawson-Saunders B, Trapp RG. Basic and Clinical
Biostatistics. 2nd ed. Norwalk, CT: Appleton & Lange; 1994.
4) Dettli LC. Drug dosage in patients with renal disease.
Clin Pharmacol Ther 1974;16:274-80.
5) Drusano LG, Munice HL, Hoopes JM et al. Commonly used
methods of estimating creatinine clearance are inadequate for
elderly debilitated nursing home patients. J Am Geriatrics
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6) Hailemeskel B, Namanny M, Kurz A. Estimating
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7) Jelliffe RW. Estimation of creatinine clearance when urine
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8) Levey AS, Greene T, Kusek JW, et al. A simplified equation
to predict glomerular filtration rate from serum creatinine (Abstr)
J Am Soc Nephrol 2000;(11):155A
9) Levey AS, Greene T, Schluchter MD, et al. Glomerular
filtration rate measurements in clinical trials. Modification
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10) Levey AS. Assessing the effectiveness of therapy to
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11) Levey AS, Bosch JP, Lewis JB, et al. A more accurate
method to estimate glomerular filtration rate from serum
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1999;130(6):461-70
12) Rhodes RS, Sims PJ, Culbertson VL et al. Accuracy of
creatinine clearance estimates in geriatric males with
elevated serum creatinine clearance. J Geriatric Drug Ther
1991;5:31-45.
13) Smythe M, Hoffman J, Kizy K et al. Estimating creatinine
clearance in elderly patients with low serum creatinine
concentrations. Am J Hosp Pharm 1994;51:189-204. |
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