Dried blood spot versus venous blood sampling for phenylalanine and tyrosine

Background: This study investigated the agreement between various dried blood spot (DBS) and venous blood sample measurements of phenylalanine and tyrosine concentrations in Phenylketonuria (PKU) and Tyrosinemia type 1 (TT1) patients. Study design: Phenylalanine and tyrosine concentrations were studied in 45 PKU/TT1 patients in plasma from venous blood in lithium heparin (LH) and EDTA tubes; venous blood from LH and EDTA tubes on a DBS card; venous blood directly on a DBS card; and capillary blood on a DBS card. Plasma was analyzed with an amino acid analyzer and DBS were analyzed with liquid chromatography-mass spectrometry. Agreement between different methods was assessed using Passing and Bablok fit and Bland Altman analyses. Results: In general, phenylalanine concentrations in LH plasma were comparable to capillary DBS, whereas tyrosine concentrations were slightly higher in LH plasma (constant bias of 6.4 μmol/L). However, in the low phenylalanine range, most samples had higher phenylalanine concentrations in DBS compared to LH plasma. Remarkably, phenylalanine and tyrosine in EDTA plasma were higher compared to all other samples (slopes ranging from 7 to 12%). No differences were observed when comparing capillary DBS to other DBS. Conclusions: Overall agreement between plasma and DBS is good. However, bias is specimen-(LH vs EDTA), and possibly concentration-(low phenylalanine) dependent. Because of the overall good agreement, we recommend the use of a DBS-plasma correction factor for DBS measurement. Each laboratory should determine their own factor dependent on filter card type, extraction and calibration protocols taking the LH plasma values as gold standard

Protein intake during hemodialysis maintains a positive whole body protein balance in chronic hemodialysis patients

Protein energy malnutrition is present in 18 to 56% of hemodialysis patients. Because hemodialysis has been regarded as a catabolic event, we studied whether consumption of a protein- and energy-nriched meal improves the whole body protein balance during dialysis in chronic hemodialysis (CHD) patients. Patients were studied on a single day between dialysis (HD- protocol) in the morning while fasting and in the afternoon while consuming six small test meals. Patients were also studied during two separate dialysis sessions (HD+ protocol). Patients were fasted during one and consumed the meals during the other. Whole body protein metabolism was studied by primed constant infusion of L-[1-C-13]valine. During HD-, feeding changed the negative whole body protein balance observed during fasting to a positive protein balance. Dialysis deepened the negative balance during fasting, whereas feeding during dialysis induced a positive balance comparable to the HD- protocol while feeding. Plasma valine concentrations during the studies were correlated with whole body protein synthesis and inversely correlated with whole body protein breakdown. We conclude that the consumption of a protein- and energy-enriched meal by CHD patients while dialyzing can strongly improve whole body protein balance, probably because of the increased amino acid concentrations in blood