Nefrologia 2012;32(1):115117  Doi. 10.3265/Nefrologia.pre2011.Dec.11235 
Estimating glomerular filtration rate in order to adjust drug doses: confusion abounds 
Estimación de la tasa de filtración glomerular para el ajuste posológico de los fármacos. Reina la confusión 
Enviado a Revisar: 15 Nov. 2011  Aceptado el: 4 Dic. 2011  En Publicación: 27 Ene. 2012 
Javier PeralAguirregoitia^{1}, Unax LertxundiEtxebarria^{2}, Ramon SarachoRotaeche^{3}, Sira IturrizagaCorrecher^{4}, M. José MartínezBengoechea^{5} 
^{1}Servicio de Farmacia Hospitalaria. Hospital GaldakaoUsansolo. Galdakao, Vizcaya (Spain) ^{2}Farmacéutico especialista en Farmacia Hospitalaria. Jefe de Servicio de Farmacia. Hospital Psiquiátrico de Álava. VitoriaGasteiz, Álava (Spain) ^{3}Servicio de Nefrología. Hospital Santiago Apóstol. VitoriaGasteiz, Álava (Spain) ^{4}Laboratorio de Análisis Clínicos. Hospital Txagorritxu. VitoriaGasteiz, Álava (Spain) ^{5}Farmacéutica especialista en Farmacia Hospitalaria. Jefa de Servicio de Farmacia. Hospital GaldakaoUsansolo. Galdakao, Vizcaya (Spain) 
Correspondencia para Javier PeralAguirregoitia, Servicio de Farmacia Hospitalaria, Hospital GaldakaoUsansolo, Barrio Labeaga s/n, 48960, Galdakao, Vizcaya, Spain Email: javier.peralaguirregoitia@osakidetza.net Email: javier.peralaguirregoitia@gmail.com 
Table 1  Recommended dabigatran dose adjustments according to the glomerular filtration rate estimated by different equations 
Table 2  Recommended daptomycin dose adjustments according to the glomerular filtration rate estimated by different equations 

To the Editor, Two recent events led to our writing this letter. One. For 2 or 3 years now, our local biochemistry laboratories calculate the estimated glomerular filtration rate (eGFR) by means of the MDRDIDMS formula (formerly MDRD) and the isolated creatinine value, as per National Kidney Foundation recommendations.^{1} Yet in October 2011, we still observe the following:  The constant used by some biochemistry laboratories for the MDRDIDMS formula is 186, when it should be 175 since the calculation for the serum creatinine value is standardised by IDMS.  Some laboratories deliver MDRDIDMS results in ml/min instead of ml/min/1.73 m^{2}. Although it is dependent on an individual’s body surface area, this could lead one to assume that the measurement is absolute, which could have consequences when adjusting doses. Two. While the new formula for measuring glomerular filtration rate (GFR),^{2} CKDEPI, seems to improve on the current MDRDIDMS formula in both accuracy and precision, it is likely to make things even more confusing when it comes to choosing an equation to adjust a drug dose. Furthermore, an article recently published in this journal comparing the MDRDIDMS and CKDEPI formulas in a Spanish population^{3} contained what appears to be an erratum in Table 1, which describes the formulas used to calculate CKDEPI: for males with creatinine levels >80 micromoles/litre, it states to divide by 0.7, and we believe that it should be by 0.9. In light of all of the above, we would like to make the following observation: From the 1980s until quite recently, GFR was estimated using the formula published by Cockcroft and Gault (CG) in 1976.^{4} The result of this equation (an estimation of creatinine clearance) was used to evaluate renal function and adjust the doses of any drugs that so required. We would like to stress that the value obtained by this formula is absolute. This means that it accounts for the individual’s size (since it includes weight among its variables) and gives a result in ml/min (if the body surface area differs greatly from the mean, using ideal rather than true weight is recommended.) In 1999, 23 years after the CG formula was published, Levey published a new formula for estimating GFR: the MDRD.^{5} Shortly afterwards, in 2002, the KDOQI proposed using this formula for early detection and classification of chronic kidney disease so that patients in earlier stages would have better access to nephrology care.^{1} The result given by this formula is dependent on body surface area (ml/min/1.73m^{2}), as is also the case with the recently improved MDRDIMDS and CKDEPI formulas.^{6,2} Since the result is dependent on a surface area of 1.73 m^{2}, we only need the variables age, sex, serum creatinine and race. This formula was recommended by such societies as the Spanish Society of Clinical Biochemistry and Molecular Pathology (SEQC) and the Spanish Society of Nephrology (S.E.N).^{7} Nevertheless, although generalised use of the MDRD method seems appropriate for categorising individuals in different stages of chronic kidney disease, it causes some problems in adjusting drug doses, especially if the value given by the laboratories is interpreted as an absolute value. If we consider only the relative results (ml/min/1.73m^{2}) given by MDRD, MDRDIMDS or CKDEPI –those results given by biochemical laboratories– individuals with a body surface area >1.73m^{2} will have a higher absolute eGFR value. This could lead to underdosing the patient. If the patient’s body surface area is less than 1.73m^{2}, the absolute eGFR will be lower, which could lead to overdosing the patient (Tables 1 and 2). On the other hand, the required dose of a certain drug may vary considerably depending on the equation used to estimate GFR, and this may have clinical repercussions.^{8} With this in mind, most published drug adjustment guidelines recommend a dose and/or drug interval according to the CockcroftGault formula; very few guidelines make use of MDRD.^{9} In two recent examples, regulatory authorities based their recommendations on the CG formula:  The Spanish Agency for Medicines and Health Products (AEMPS) followed the European Medicines Agency recommendation and modified the SmPC for Pradaxa® (dabigatran) and issued an informative note on 27 October 2011 reminding doctors of the importance of checking renal function before and after treatment with this new drug. They informed that before starting dabigatran treatment, renal function must be assessed in all patients by calculating creatinine clearance (CrCl) in order to exclude patients with severe renal failure (CrCl<30ml/min).^{10}  The Food and Drug Administration (FDA)’s safety update of 1 September 2011 stated that the SmPC had been changed and issued a reminder that “Reclast should not be used (is contraindicated) in patients with creatinine clearance less than 35ml/min”.^{11} The FDA guidelines for the industry simply cite the CG and MDRD equations as being the most commonly used. However, experts do not agree on which of the formulas should be used for adjusting doses in patients with renal failure. Some advocate using the equation recommended by the pharmaceutical manufacturer, particularly in the case of elderly patients,^{12} while others^{13,14} state that the MDRD and CG equations are completely interchangeable. In summary, and as a general rule, using the equation recommended by the pharmaceutical manufacturer (mainly CG) seems reasonable. If there is no specific recommendation, the most reliable method of estimating GFR in the target population should be should. Regardless of which equation is used, we must remember that dose adjustments must be made using absolute GFR values, especially for patients whose body surface area differs greatly from 1.73m^{2}. 
Referencias Bibliográficas 
1. National Kidney Foundation. K/DOQI clinical practice guidelines for chronic kidney disease: evaluation, classification, and stratification. Am J Kidney Dis 2002;39(2 Suppl 1):S1266.[Pubmed] 2. Levey AS, Stevens LA, Schmid CH, Zhang YL, Castro III AF, Feldman HI, et al., for the CKDEPI (Chronic Kidney Disease Epidemiology Collaboration). A new equation to estimate glomerular filtration rate. Ann Intern Med 2009;150(9):60412.[Pubmed] 3. Montañés R, Bover J, Oliver A, Ballarín JA, Gracia S. Valoración de la nueva ecuación CKDEPI para la estimación del filtrado glomerular. Nefrologia 2010;30(2):18594.[Pubmed] 4. Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron 1976;16(1):3141.[Pubmed] 5. Levey AS, Bosch JP, Lewis JB, Greene T, Rogers N, Roth D. A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group. Ann Intern Med 1999;130(6):46170.[Pubmed] 6. Levey AS, Coresh J, Greene T, Marsh J, Stevens LA, Kusek JW, et al. Expressing the Modification of Diet in Renal Disease Study equation for estimating glomerular filtration rate with standardized serum creatinine values. Clin Chem 2007;53(4):76672.[Pubmed] 7. Gracia S, Montañés R, Bover J, Cases A, Deulofeu R, Martín de Francisco AL, et al. Recomendaciones sobre la utilización de ecuaciones para la estimación del filtrado glomerular en adultos. Nefrologia 2006;26:65865.[Pubmed] 8. Denetclaw TH, Oshima N, Dowling TC. Dofetilide dose calculation errors in elderly associated with use of the modification of diet in renal disease equation. The Ann Pharmacother 2011;45:e44.[Pubmed] 9. Dowling TC, Matzke GR, Murphy JE, Burckart GJ. Evaluation of renal drug dosing: prescribing information and clinical pharmacist approaches. Pharmacotherapy 2010;30(8):776786.[Pubmed] 10. Nota informativa de la AEMPS, del 27 de octubre de 2011, sobre dabigatrán (Pradaxa®) y riesgo de hemorragia: nuevas recomendaciones de vigilancia de la función renal. Available at: http://www.aemps.gob.es/informa/notasInformativas/medicamentosUsoHumano/seguridad/2011/NIMUH_212011.htm. [Accessed: Oct/31/2011]. 11. Comunicado de la FDA, del 1 de septiembre de 2011, sobre la seguridad de los medicamentos: Nueva contraindicación y advertencia actualizada sobre el deterioro renal causado por Reclast (ácido zoledrónico). Available at: http://www.fda.gov/Drugs/DrugSafety/ucm270199.htm#ref. [Accessed: Oct/31/2011]. 12. Corsonello A, Pedone C, Lattanzio F, Semeraro R, D'Andria F, Gigante M, et al. Agreement between equations estimating glomerular filtration rate in elderly nursing home residents and in hospitalised patients: implications for drug dosing. Age and Ageing 2011;40:5839.[Pubmed] 13. Stevens LA, Nolin TD, Richardson MM, Feldman HI, Lewis JB, Rodby R, et al., on behalf of the Chronic Kidney Disease Epidemiology Collaboration (CKDEPI). Comparison of drug dosing recommendations based on measured GFR and kidney function estimating equations. Am J Kidney Dis 2009;54:3342.[Pubmed] 14. Jones G. Estimating renal function for drug dosing decisions. Clin Biochem Rev 2011;32:818.[Pubmed] 