Comparing different statistical methods for evaluating diagnostic effectiveness of clinical tests: respiratory distress syndrome as a model.

Phospholipids in specimens of amniotic fluid from 346 patients were quantified and the results evaluated in light of the clinical outcome. Fifty-eight neonates had respiratory distress syndrome. We used this data base to compare different statistical methods for evaluating test effectiveness and diagnostic discrimination. Dichotomizing quantitative tests into binary tests with arbitrary cutoff values was inadequate for comparing test effectiveness. Subgrouping the data into deciles and calculating the incidence of respiratory distress syndrome for each decile avoided the problems of the preceding approach and was easy to calculate and comprehend; however, this method lacked statistical power. Relative operating characteristic curves yielded more statistical power, but results were more difficult to calculate and were not intuitively obvious to most workers in the laboratory. A modified cumulative frequency plot, combining elements of both decile subgrouping and relative operating characteristic curves, was easily calculated and intuitively obvious. These plots, like relative operating characteristic curves, provided an index for quantifying test effectiveness. When used in combination with standard cumulative frequency curves, they also provided direct diagnostic information on disease probability for any value of the clinical assay.

Evaluating the clinical effectiveness of amniotic fluid assays in predicting respiratory distress syndrome in the neonate.

Amniotic fluid phospholipids from 346 patients' specimens were quantified and evaluated against the clinical outcome (i.e., respiratory distress syndrome or normal respiratory function). Concentrations of lecithin, sphingomyelin, phosphatidylglycerol, and the lecithin/sphingomyelin reflectance ratio were evaluated by ordered frequency distribution and stepwise discriminant function analysis. The lecithin/sphingomyelin ratio was the best single test for discriminating between respiratory distress syndrome and normal pulmonary function in the fetus, slightly superior to lecithin assay alone. A combination of lecithin/sphingomyelin ratio and lecithin concentration, however, appeared to optimize the discriminant function, although the clinical significance of this test combination remained marginal. High concentrations of phosphatidylglycerol were correlated with high concentrations of lecithin, and virtually ruled out respiratory distress syndrome. Absence of phosphatidylglycerol was not diagnostic. High concentrations of sphingomyelin increased the probability of respiratory distress syndrome. We suggest the following stepwise series of tests to optimize diagnosis: phosphatidylglycerol concentration, sphingomyelin concentration, and finally lecithin-sphingomyelin ratio.