Evaluation of failure to thrive: diagnostic yield of testing for renal tubular acidosis.

BACKGROUND Failure to thrive (FTT) poses a diagnostic dilemma for pediatricians. The kidney disorder that is considered most often is renal tubular acidosis (RTA). However, the prevalence of RTA may be overestimated, leading to unnecessary referrals for subspecialty evaluation. Moreover, preliminary data suggest that venous blood gas (VBG) testing may provide a more accurate measurement of the serum bicarbonate concentration than routine biochemical testing. OBJECTIVE 1) To compare the results of bicarbonate measurements by using VBG and routine clinical biochemistry methods under a variety of in vitro conditions; 2) to determine the frequency of RTA as a cause of FTT; and 3) to assess the utility of VBG measurement of serum bicarbonate in the clinical assessment of RTA. EXPERIMENTAL DESIGN AND METHODS: In blood samples collected from healthy volunteers, bicarbonate was measured by using a VBG apparatus and a clinical biochemistry analyzer under a variety of in vitro conditions including variation in time until sample processing (0-4 hours), variations in temperature (room temperature versus on ice), addition of sodium fluoride, or addition of heparin. A retrospective chart review was also performed of all children referred to the renal clinic for evaluation of FTT during the 5-year period of 1997 to 2002. The following data were collected for each case: demographic and clinical information and laboratory testing including serum bicarbonate determined by both routine biochemical testing and VBG analysis. Data are reported as mean +/- standard deviation. RESULTS In the in vitro studies, VBG determination of bicarbonate concentration consistently yielded a value that was 3 to 6 mmol/L higher than routine biochemical analysis regardless of whether the sample was processed immediately or up to 4 hours later, maintained at room temperature or on ice until the measurement was performed, or the sample tube contained sodium fluoride or heparin. Thirty-six children were referred to exclude a renal etiology of FTT with a presumptive diagnosis of RTA in all cases. The patient group was comprised of 16 males and 20 females whose ages ranged from 4 to 156 months (mean: 27 +/- 33 months). The serum bicarbonate concentration determined by biochemical testing was 18 +/- 4 mmol/L, whereas the bicarbonate level by VBG was 24 +/- 3 mmol/L. The mean difference between the bicarbonate by VBG and bicarbonate by routine biochemistry measurements was 5.6 +/- 4.4. Only 1 child (2.8%) was confirmed to have RTA. CONCLUSIONS RTA is a rare renal cause of FTT in children. VBG determination of serum bicarbonate yielded a significantly higher value than the result obtained by routine biochemical testing under both in vitro and in vivo conditions. These data suggest that reliance on routine biochemical testing results in an overestimation of the importance of RTA as a cause of FTT. We recommend the use of a VBG determination of serum bicarbonate concentration for the evaluation of a child with FTT who is thought to have a metabolic acidosis. Adoption of this practice will reduce the number of children suspected of having RTA and decrease the need for referral to a nephrologist for further evaluation.