Sir.-Maynard and colleagues1 discuss a very timely and clinically important issue, ie, detection of drugs of abuse in pregnancy and in the newborn. Their study describes the use of meconium samples, which are easier to collect than urine and provide information about drugs that may have been used for at least several weeks prior to delivery. However, the authors state that "meconium testing predicted a positive urine test in 14 of 17 cases ... and predicted a negative urine test in 10 of 11 cases ..." They refer to these as the positive and negative predictive values of the test.
Positive predictive value is defined as the "probability of disease in a patient with a positive (abnormal) test result." Negative predictive value is defined as the "probability of not having the disease when the test result is negative (normal)."2 To determine whether the patient has the disease (ie, whether the baby has been