title: Comparison of iothalamate clearance methods for measuring GFR.
abstract: OBJECTIVE
  To evaluate the bias and precision of three methods of measuring glomerular filtration rate (GFR) relative to a standard method.
  
  
  METHODS
  Prospective, outpatient study.
  
  
  METHODS
  University-affiliated general clinical research center.
  
  
  METHODS
  Twenty-six patients with various degrees of renal function (GFR range 25-151 ml/min/1.73 m2).
  
  
  METHODS
  Each patient received iothalamate twice during the study visit, first as a bolus injection and then as a priming dose followed by a constant-rate infusion for 2.5 hours.
  
  
  RESULTS
  Plasma (ClpIVB) and renal clearances (ClrIVB) after bolus injection and plasma clearance during constant-rate infusion (ClpINF) were compared with standard renal clearance during constant-rate infusion (ClrINF). All three measures were highly correlated with ClrINF (r>0.90, p<0.001). The mean ClrIVB was not significantly different from ClrINF (106.3+/-30.4 vs 104.2+/-28.5 ml/min/1.73 m2) and provided a precise (8.8%, 95% CI 6.5-11.1%) and unbiased measure of GFR. Both ClpIVB and ClpINF were positively biased; values exceeded ClrINF by 11.8+/-11.1 (p=0.0001) and 10.5+/-12.5 ml/min/1.73 m2 (p=0.0003), respectively. Use of a nonrenal correction factor of 9.8 and 10.5 ml/min/1.73 m2 for infusion and bolus plasma clearance values, respectively, eliminated bias and improved the precision of these methods.
  
  
  CONCLUSIONS
  Iothalamate renal clearance after bolus injection is a simple, accurate, and precise measurement of GFR and may be a useful alternative to the standard infusion method in clinical investigations. The corrected plasma clearance provides a simple index of GFR for clinical practice.
authors.affiliation: Department of Pharmaceutical Sciences, School of Pharmacy, University of Pittsburgh, Pennsylvania 15261, USA.
authors.forename: T C
authors.initials: TC
authors.lastname: Dowling
authors.forename: R F
authors.initials: RF
authors.lastname: Frye
authors.forename: D S
authors.initials: DS
authors.lastname: Fraley
authors.forename: G R
authors.initials: GR
authors.lastname: Matzke
mesh_terms: D000328:Adult; D000368:Aged; D003287:Contrast Media / Q000493:pharmacokinetics*; D005260:Female; D005919:Glomerular Filtration Rate / Q000502:physiology*; D006801:Humans; D007483:Iothalamic Acid / Q000493:pharmacokinetics; D008297:Male; D008875:Middle Aged; D010045:Outpatients; D011446:Prospective Studies
chemical_list: D003287:Contrast Media; D007483:Iothalamic Acid