Measures of urine concentration in maternal urine screening for Down syndrome.

A study was carried out to assess eight methods of normalizing the level of urinary beta-core human chorionic gonadotropin (hCG) for variable urine concentration. We compared the standard approach--creatinine determination by the Jaffe method--with high performance liquid chromatography (HPLC) measurement of creatinine, osmolarity and optical density at five wavelengths. Urine samples were included from a total of 472 women with unaffected singleton pregnancies at 15 weeks' gestation. The median beta-core hCG value was determined for each decile group when the results were ranked in turn according to the different measures of urine concentration. Creatinine using the Jaffe method had a much stronger relationship with median beta-core hCG than the other measures. Linear regression across the decile groups gave an R2 value for Jaffe of 0.85 compared with HPLC of 0.53, osmolarity of 0.52, optical density at 405 nm of 0.72, at 450 nm of 0.57, at 490 nm of 0.33, at 570 nm of 0.34 and at 630 nm of 0.33. We conclude that when screening with urinary beta-core hCG measuring creatinine appears to be an adequate method of allowing for variable urine concentration.