Use of the phenylalanine:tyrosine ratio to test newborns for phenylketonuria in a large public health screening programme.

OBJECTIVE: To assess the benefits of using the phenylalanine:tyrosine ratio to screen newborns for phenylketonuria (PKU). SETTING: Data were collected from all newborns in California during a ten month period (n = 404,381). METHODS: Dried blood spot specimens were analysed at nine laboratories. To assure that the results reported from multiple sites were matched accurately, an automated methodology was chosen that included sample processing, analysis, telecommunications, reporting, and information technology. Phenylalanine and tyrosine concentrations were measured independently by continuous flow fluorometry, for which precision, recovery, detection limits, carryover, chemical specificity, reportable range, and number of repeats are reported. RESULTS: In this study, 37% of the newborns were tested at less than 24 hours of age. For this population, using a phenylalanine only cut off of 200 mumol/l, there were 48 recalled infants per case of classic PKU. Using the phenylalanine:tyrosine ratio with a cut off of 1.50, screen positives could be reported with phenylalanine as low as 150 mumol/l and with only 12 recalls per case. CONCLUSIONS: The phenylalanine:tyrosine ratio can be measured accurately at multiple laboratories using two channel chemical analyses. Having applied the methods to the routine clinical screening of a large population, it was confirmed that the clinical sensitivity and specificity of the PKU screening test are higher when the phenylalanine:tyrosine ratio is incorporated into the cut off than when the cut off is based on the phenylalanine concentration alone.

Automated HPLC screening of newborns for sickle cell anemia and other hemoglobinopathies.

Automated HPLC is used to test dried blood-spot specimens from newborns for hemoglobins (Hb) F, A, S, C, E, and D. We present the method and report on its performance determined during >4 years of testing 2.5 x 10(6) newborns. The method features automated derivation of presumptive phenotypes; quantitative quality control and proficiency testing; throughput of one specimen per minute; small sample volume; hemoglobin concentrations quantified with an interlaboratory CV of 14-18%; retention times with interlaboratory CV of <2% and matching, within +/- 0.03 min, of laboratories and reagent lots; control of peak resolution; 0.5% detection limit for Hb S and C, and 1.0% for Hb F, A, E, and D; few interferences; and negligible background and carryover. Shortcomings of the method are the absence of microplate barcode identification and the need for manually pipetting the sample eluate into the microplate.

Hypertensive crisis and death associated with phencyclidine poisoning.

Phencyclidine hydrochloride is a dangerous drug. Its incidence as the causative agent in childhood poisoning is increasing. A pressor effect of phencyclidine has been noted in studies both in man and in experimental animals. We summarize seven cases of poisoning with this drug, including one in which death occurred following a hypertensive crisis. Patients who have ingested this drug should have continuous monitoring of blood pressure in an intensive care unit.

Standardization of fluorescence spectra and the calibration of spectrofluorimeters.

A technique is proposed that can be used to calibrate the amplitude of fluorescence spectra so that spectra reported by different laboratories can be compared quantitatively. The technique is based upon the scattering of light by a solution of polystyrene. In order to compare quantitatively data obtained from different spectrofluorimeters, these terms are defined: the spectrofluorimetric sensitivity, the noise-equivalent concentration, and the fluorophotometric reproducibility. The author shows how these quantities can be measured with the polystyrene standard.