Immunoreactive trypsin on dried-blood spots as a possible neonatal test for cystic fibrosis. (I. Evaluation of the method and preliminary field trial)

Determination of amino acids by ion-exchange chromatography on filter paper spotted blood samples stored at different temperatures and for different periods: comparison with capillary and venous blood

OBJECTIVE

To determine whether the well-established filter paper spotted blood method used for the determination of some amino acids could be reliably used to measure all amino acids and whether amino acid results thus obtained are reproducible and comparable to the results obtained by measuring plasma amino acids in either capillary or venous blood.

METHODS

This is a prospective study in which blood samples from a finger-prick were collected in capillary tubes and at the same time blotted on filter papers; another sample was taken from a vein, from 19 healthy volunteers aged between 18 and 24 yr after a strict 12-h overnight fast. Another 9 healthy adult volunteers provided blood samples on filter papers for the storage study; 9 samples were analyzed immediately; 9, 8 and 4 samples were stored at -20 degrees C, -4 degrees C and room temperature respectively and analyzed after 14 days; 8 samples stored at -20 degrees C were analyzed after 4 weeks.

RESULTS

Intra-sample reproducibility in the filter paper blood from the same individual was found to be mostly less than 20%, while for the capillary blood was less than 5%. The greatest variability was in cystine and methionine. There was no significant difference between results obtained from capillary blood and from venous blood, but there was a significant difference between amino acid concentrations in venous and capillary blood on the one hand and filter paper blood on the other. Storage at different temperatures and for a varied period of time showed little change except in serine, glutamate, ornithine, histidine, cystine and methionine. There was a 30% decrease in concentrations of most amino acids in filter paper blood when compared to capillary or venous blood probably because of loss in the extraction process.

CONCLUSION

A new set of values for amino acids in filter paper blood in normal individuals is presented. Blood spotted filter paper could be used to screen practically all inborn errors of amino acid metabolism.

Cystic fibrosis screening by dried blood spot trypsin assay: results in 75,000 newborn infants

Seventy-five thousand 5-day-old babies were screened for cystic fibrosis by blood spot immunoreactive trypsin (IRT) assay as part of a statewide screening program. IRT was elevated in 433 babies; retesting revealed persistent elevation in 38. Sweat testing confirmed cystic fibrosis in 35 babies and was normal in two babies, whose IRT remained elevated at the time of the test. Sweat testing was refused by one mother. Of the 35 babies with cystic fibrosis, 13 had meconium ileus or an already diagnosed affected sibling, but the diagnosis was unsuspected in 22, although all but four had some symptoms suggestive of cystic fibrosis. Stool trypsin activity at the time of the diagnostic screen was normal in nine and reduced in seven of the babies with cystic fibrosis. One baby did not have elevated IRT, and the cystic fibrosis was missed by the screening test. In a retrospective study of blood spot samples from 36 newborn infants, who were later diagnosed as having cystic fibrosis, all had IRT levels greater than in matched controls. Our study confirms that elevated IRT is characteristic of newborn babies with cystic fibrosis, and shows that this test is very specific and sensitive when used as a newborn screening test.