serum creatinine < 0.8 mg/dl, use 0.8 mg/dl for serum creatinine. schwartz formula in children, the schwartz formula is used. this employs the serum creatinine (mg/dl), the child's height (cm) and a constant to estimate the glomerular filtration rate: egfr = k × height serum creatinine =\times where k is a constant that depends on muscle mass, which itself varies with a child's age: in first year of life, for pre-term babies k=0.33 and for full-term infants k=0.45 for infants and children of age 1 to 12 years, k=0.55. the method of selection of the constant k has been questioned as being dependent upon the gold-standard of renal function used (i.e. inulin clearance, creatinine clearance, etc.) and also may be dependent upon the urinary flow rate at the time of measurement. in 2009 the formula was updated to use standardized serum creatinine (recommend k=0.413) and additional formulas that allow improved precision were derived if serum cystatin c is measured in addition to serum creatinine. idms standardization effort one problem with any creatinine-based equation for gfr is that the methods used to assay creatinine in the blood differ widely in their susceptibility to non-specific chromogens, which cause the creatinine value to be overestimated. in particular, the mdrd equation was derived using serum creatinine measurements that had this problem. the nkdep program in the united states has attempted to solve this problem by trying to get all laboratories to calibrate their measures of creatinine to a "gold standard", which in this case is isotope dilution mass spectrometry (idms). in late 2009 not all labs in the u.s. had changed over to the new system. there are two forms of the mdrd equation that are available, depending on whether or not creatinine was measured by an idms-calibrated assay. the ckd-epi equation is designed to be used with idms-calibrated serum creatinine values only. normal ranges the normal range of gfr, adjusted for body surface area, is 100–130 average 125 ml/min/1.73m2 in men and 90–120 ml/min/1.73m2 in women younger than the age of 40. in children, gfr measured by inulin clearance is 110 ml/min/1.73 m2 until 2 years of age in both sexes, and then it progressively decreases. after age 40, gfr decreases progressively with age, by 0.4–1.2 ml/min per year. decreased gfr a decreased renal function can be caused by many types of kidney disease. upon presentation of decreased renal function, it is recommended to perform a history and physical examination, as well as performing a renal ultrasound and a urinalysis. the most relevant items in the history are medications, edema, nocturia, gross hematuria, family history of kidney disease, diabetes and polyuria. the most important items in a physical examination are signs of vasculitis, lupus erythematosus, diabetes, endocarditis and hypertension. a urinalysis is helpful even when not showing any pathology, as this finding suggests an extrarenal etiology. proteinuria and/or urinary sediment usually indicates the presence of glomerular disease. hematuria may be caused by glomerular disease or by a disease along the urinary tract. the most relevant assessments in a renal ultrasound are renal sizes, echogenicity and any signs of hydronephrosis. renal enlargement usually indicates diabetic nephropathy, focal segmental glomerular sclerosis or myeloma. renal atrophy suggests longstanding chronic renal disease. chronic kidney disease stages main article: chronic kidney disease risk factors for kidney disease include diabetes, high blood pressure, family history, older age, ethnic group and smoking. for most patients, a gfr over 60 ml/min/1.73m2 is adequate. but significant decline of the gfr from a previous test result can be an early indicator of kidney disease requiring medical intervention. the sooner kidney dysfunction is diagnosed and treated the greater odds of preserving remaining nephrons, and preventing the need for dialysis. ckd stage gfr level (ml/min/1.73 m2) stage 1 ≥ 90 stage 2 60–89 stage 3 30–59 stage 4 15–29 stage 5 < 15 the severity of chronic kidney disease (ckd) is described by six stages; the most severe three are defined by the mdrd-egfr value, and first three also depend on whether there is other evidence of kidney disease (e.g., proteinuria): 0) normal kidney function – gfr above 90 ml/min/1.73 m2 and no proteinuria 1) ckd1 – gfr above 90 ml/min/1.73 m2 with evidence of kidney damage 2) ckd2 (mild) – gfr of 60 to 89 ml/min/1.73 m2 with evidence of kidney damage 3) ckd3 (moderate) – gfr of 30 to 59 ml/min/1.73 m2 4) ckd4 (severe) – gfr of 15 to 29 ml/min/1.73 m2 5) ckd5 kidney failure – gfr less than 15 ml/min/1.73 m2 some people add ckd5d for those stage 5 patients requiring dialysis; many patients in ckd5 are not yet on dialysis. note: others add a "t" to patients who have had a transplant regardless of stage. not all clinicians agree with the above classification, suggesting that it may mislabel patients with mildly reduced kidney function, especially the elderly, as having a disease. a conference was held in 2009 regarding these controversies by kidney disease: improving global outcomes (kdigo) on ckd: definition, classification and prognosis, gathering data on ckd prognosis to refine the definition and staging of ckd.