Manual. Transfer step. Figure 1. Workflow comparison of manual phenylalanine assay and automated GSP Neonatal Phenylalanine assay

Transcription

1 TECHNICAL NOTE GSP Neonatal Phenylalanine kit / B GSP Neonatal Phenylalanine kit ( / B) is a fully Introduction automated enzymatic assay for the GSP system. Optimized for the quantitative determination of phenylalanine concentration in blood specimens dried on filter paper, the kit is an effective aid in screening newborns for phenylketonuria (Figure 1.). Analytical performance of the GSP Neonatal Phenylalanine kit was determined in verification studies conducted at PerkinElmer, Turku, Finland. In order to establish population distribution data and screening performance for the GSP Neonatal Phenylalanine kit two studies were conducted in newborn screening laboratories. In these studies GSP Phenylalanine kit was compared to manual Phenylalanine kit (NP-1000/-4000). Punch Reagent preparation Pipetting & incubation Transfer step Pipetting & incubation Measurement Result GSP Punch Reagent preparation Load Result Figure 1. Workflow comparison of manual phenylalanine assay and automated GSP Neonatal Phenylalanine assay Key benefits over existing manual assays: Automated assay Only two hands-on steps for reconstitution 24 hrs valid calibration curve Control step to detect missing sample disks Resorufin step added in the reaction to decrease the unspecific background fluorescence All reagents and QC material are bar-coded Simultaneous screening of any other GSP assays in any order Reagent on-board stability 4 days / 4 plates

2 Full selection of reagents in one kit Two different sizes of kits are available as 12 plates kit ( ) and a bulk kit for 60 plates [ B] Neonatal Phenylalanine Calibrators - 7 [15] dried blood spot cassettes each containing 1 set of dried blood spots Neonatal Phenylalanine Controls - 4 [20] dried blood spot cassettes each containing 3 sets of dried blood spots Neonatal Phenylalanine Substrate Reagent - 3 [15] vials, lyophilized Neonatal Phenylalanine Substrate Reagent - 3 [15] vials, lyophilized Neonatal Phenylalanine Assay Buffer - 3 [15] bottles, ready-for-use Neonatal Extraction Solution - 1 [5] bottles, ready-for-use Barcode labels for the plates Validated assay method The GSP Neonatal Phenylalanine assay is based on the fluorescent phenylalanine dehydrogenase method, which differs from the fluorescent ninhydrin method used in manual Neonatal Phenylalanine kit (NP-1000/NP-4000). In the first reaction phenylalanine dehydrogenase converts phenylalanine in sample to phenylpyruvate generating NADH. In the presence of NADH, resazurin dye is reduced to fluorescent resorufin, which is measured using an excitation wavelength of 505 nm and an emission wavelength of 580 nm (Figure 3.). Due to longer excitation wavelength than with manual phenylalanine assay, unspecific background fluorescence is reduced for improved performance. GSP Neonatal Phenylalanine kit assay protocol After punching of the samples and reconstitution of the Phenylalanine Substrate and Enzyme Reagents, the assay is fully automated from plate loading to completion. The assay time for one plate is 2 h 28 min and around 2-3 plates can be processed per hour after an initial 3 hour dwell time (Figure 4.). Dispensing Extraction Solution (20 µl/well) Incubation (1 h) Dispensing Assay Buffer (100 µl/well) Dispensing Substrate Reagent (5 µl/well) Dispensing Enzyme Reagent (5 µl/well) Incubation (1 h) Measurement (Phe concentration) Measurement (Elution Control to check the sample disk is present in a well) Figure 3. The assay principle of the GSP Neonatal Phenylalanine kit Figure 2. Content of the GSP Neonatal Phenylalanine kit for 12 plates ( ) Figure 4. Run scenario for GSP Neonatal Phenylalanine assay protocol. 2 3

3 Assay Performance Calibration The primary calibrator of the kit is the 3rd ISNS Reference preparation for Neonatal screening. The calibration unit for phenylalanine concentration is defined as µmol/l (or mg/dl) of whole blood. Conversion factor is 1 µmol/l blood= mg/dl blood (1 mg/dl=60.61 µmol/l). The kit calibrators cover the clinically relevant concentration range and the reagents include controls for low (150 µmol/l) and high (700 µmol/l) phenylalanine concentration. A typical calibration curve is shown in Figure 5. The calibration curve is valid for 24 hours. Precision The precision of the assay was determined in accordance with CLSI document EP05-A2 [1] using dried blood spot samples, 3 kit lots and 3 GSP instruments. An analysis of variance approach was used to calculate the variance components. The within-run, within-lot and total variation results for the GSP Neonatal Phenylalanine kit are presented in Table 1. Lower limits of detection, measuring range and linearity The analytical limits were determined in accordance with CLSI document EP17-A2 [2] and linearity was determined in accordance with CLSI document EP06-A [3]. Analytical limits, measuring range and linearity of the GSP Neonatal Phenylalanine kit are summarized in Table 2. The Limit of Blank (LoB) is defined as the 95th percentile of a distribution of blank samples (n=150), the Limit of Detection (LoD) is based on 216 determinations of low level samples and the Limit of Quantitation (LoQ) is defined as the lowest concentration with a total CV equal to or less than 24%. Table 2. Analytical limits, measuring range and linearity LoB LoD LoQ Measuring range Linearity Specimen stability The influence of storage time, temperature, and humidity on phenylalanine concentration was studied using several artificial samples spiked with phenylalanine. Phenylalanine concentrations were measured at different time points after different storage conditions. Storage under desiccated conditions is recommended (Figure 6.), [4] µmol/l (0.7 mg/dl) 68 µmol/l (1.1 mg/dl) 68 µmol/l (1.1 mg/dl) from 68 µmol/l to 1200 µmol/l (from 1.1 mg/dl to 19.8 mg/dl) from LoQ to CalF from 45 µmol/l to 1420 µmol/l (from 0.7 mg/dl to 23.4 mg/dl) Figure 5. A typical calibration curve for GSP Neonatal Phenylalanine assay. The approximate concentration values for controls are shown as red crosses and relevant clinical range is highlighted. Table 1. Variation of GSP Neonatal Phenylalanine kit using one calibration curve valid for 24 hours. The clinically relevant range is highlighted. Sample n phenylalanine concentration µmol/l (mg/dl) Within lot variation Between lot variation CV% CV% CV% (0.9) (1.1) (1.5) (2.2) (7.0) (10.7) (16.6) Total variation Figure 6. The change of phenylalanine concentration during storage at different temperatures and humidity conditions over time (% of reference without storage at the zero time point). 4 5

4 Distribution of results Internal and external studies were conducted to produce normal newborn population distribution data for the GSP Neonatal Phenylalanine kit, to assess the screening performance and to compare GSP Neonatal Phenylalanine assay to manual Phenylalanine assay (NP-1000/-4000). External study 1 Phenylalanine concentration of 2064 newborn screening specimens and 22 archived confirmed high phenylalanine specimens were measured using GSP Neonatal Phenylalanine kit in a European newborn screening laboratory. Population range, mean, median and cut-off values corresponding to 99th, 99.5th and 99.9th percentile were calculated. The frequency distributions of the newborn dried blood spot specimens of measured phenylalanine concentrations are visualised in Figure 7. and descriptive statistics in Table 3. The 22 positive samples consist of 7 PKU cases with classical phenylketonuria or deficient cofactor (PKU) and 15 hyperphenylalaninemia (HPA) cases. All of the 22 retrospective high phenylalanine specimens were classified as screening positive by the GSP Neonatal Phenylalanine kit by using the higher 99th percentile and 99.5th percentile cut-off values. Figure 7. Frequency distribution of the phenylalanine concentrations measured in external study site 1 with GSP Neonatal Phenylalanine kit. Table 3. Range, mean, median values and upper percentiles for the GSP Neonatal Phenylalanine kit ( /-B) carried out in an external study 1. Figure 9. Frequency distribution of the PKU concentrations measured in an internal study with GSP Neonatal Phenylalanine kit and manual Neonatal Phenylalanine kit (NP-1000/-4000). PKU data is shown using HPLC calibration. External Study 1 Sample Size Min Max Upper percentiles ( µmol/l blood) 99% 99.5% 99.9% Routine samples Retrospective positive * Internal study An internal study comprising of 2212 leftover routine newborn screening DBS samples and 13 confirmed PKU positive newborn specimens was conducted at PerkinElmer, Turku, Finland. Phenylalanine concentration was measured with both the GSP Neonatal Phenyalalanine kit ( /-B) and manual Neonatal Table 5. Range, mean and median values with upper percentiles for the routine and retrospective positive samples using both GSP Neonatal Phenylalanine kit and manual PKU in internal study (excluding confirmed positive PKU specimens). Phenylalanine kit (NP-1000/-4000). All the positive samples were classified as screening positive using higher 99.9% percentile. The frequency distribution for routine specimens (excluding confirmed PKU specimens) is shown in Figure 9. and Table 5. External study routine newborn screening dried blood spot specimens and 37 archived confirmed high phenylalanine specimens were included in an external study 2 conducted in China and the phenylalanine concentration was measured with GSP Neonatal Phenylalanine assay. The frequency distributions of the newborn specimens for PKU concentrations are visualised in Figure 8. and descriptive statistics in Table 4. Platform Routine samples (n=2212) Confirmed positive (n=13) Upper percentiles (µmol/l blood) Min Max Min Max 95% 99% 99.5% 99.9% GSP PKU * > PKU (HPLC calibration) PKU (ISNS Calibration) * > * > *This sample was drawn from an HPA patient. The lowest sample result with classical PKU or deficient cofactor was µmol/l. Table 4. Range, mean and median values with upper percentiles for the routine and retrospective positive samples using GSP Neonatal Phenylalanine kit of external study 2 Sample type Sample Size Min Fixure 8. Frequency distribution of the PKU concentrations measured in an external study 2 with GSP Neonatal Phenylalanine kit. The x-axis is truncated to 200 µmol/l. Max Upper percentiles (µmol/l blood) 95% 99% 99.5% 99.9% Routine samples * * Retrospective positive >1400 *Sample size is too small to calculate accurate cut-off value 6 7

5 Method comparison GSP Neonatal Phenylalanine kit is calibrated against ISNS Reference preparation for Neonatal Screening [5], whereas manual PKU kit is calibrated against HPLC calibration. The method comparison data from internal verification study shows that there is a level difference between GSP and manual Phenylalanine assays (Figure 10) and no conversion factor between the methods is available. At GSP Neonatal Phenylalanine concentration levels below 120 µmol/l (or <2 mg/dl) the manual PKU results are ~15 % higher results than the GSP method, whereas at GSP Neonatal Phenylalanine concent t are ~15 % lower results than the GSP method. Result 95% Confidence Iimit Intercept to Slope to 0.81 Figure 10. Deming regression analysis results between the GSP and manual Phenylalanine methods using GSP Neonatal Phenylalanine as a reference. References [1] CLSI (2004): Evaluation of Precision Performance of Quantitative Measurement Methods; Approved Guideline Second Edition; CLSI Document EP05-A2. Wayne, PA: Clinical and Laboratory Standards Institute. [2] CLSI (2012): Evaluation of Detection Capability for Clinical Laboratory Measurement procedures; Approved Guideline Second Edition; CLSI Document EP17-A2. Wayne, PA: Clinical and Laboratory Standards Institute. [3] CLSI (2003): Evaluation of the Linearity of Quantitative Measurement Procedures: A Statistical Approach; Approved Guideline. CLSI document EP06-A. Wayne, PA: Clinical and Laboratory Standards Institute. [4] Adam, B.W., et al. (2011): The stability of markers in dried-blood spots for recommended newborn screening disorders in the United States. Clin Biochem 44, [5] Dhondt J.L., et al. (1998): Preparation of the first European working standard for phenylalanine determination in dried blood spots. J Med Screen 5, The product is not available in the USA, Canada, Russia and some Asian and Latin American countries. For a complete listing of our global offices, visit Copyright 2015, PerkinElmer, Inc. All rights reserved. PerkinElmer is a registered trademark of PerkinElmer, Inc. All other trademarks are the property of their respective owners TCH GSP Neonatal Phenylalanine kit