Investigation of the relationship between phenylalanine in venous plasma and capillary blood using volumetric blood collection devices.

Measurement of plasma and dried blood spot (DBS) phenylalanine (Phe) is key to monitoring patients with phenylketonuria (PKU). The relationship between plasma and capillary DBS Phe concentrations has been investigated previously, however, differences in methodology, calibration approach and assumptions about the volume of blood in a DBS sub-punch has complicated this. Volumetric blood collection devices (VBCDs) provide an opportunity to re-evaluate this relationship. Paired venous and capillary samples were collected from patients with PKU (n = 51). Capillary blood was collected onto both conventional newborn screening (NBS) cards and VBCDs. Specimens were analysed by liquid-chromatography tandem mass-spectrometry (LC-MS/MS) using a common calibrator. Use of VBCDs was evaluated qualitatively by patients. Mean bias between plasma and volumetrically collected capillary DBS Phe was -13%. Mean recovery (SD) of Phe from DBS was 89.4% (4.6). VBCDs confirmed that the volume of blood typically assumed to be present in a 3.2 mm sub-punch is over-estimated by 9.7%. Determination of the relationship between plasma and capillary DBS Phe, using a single analytical method, common calibration and VBCDs, demonstrated that once the under-recovery of Phe from DBS has been taken into account, there is no significant difference in the concentration of Phe in plasma and capillary blood. Conversely, comparison of plasma Phe with capillary DBS Phe collected on a NBS card highlighted the limitations of this approach. Introducing VBCDs for the routine monitoring of patients with PKU would provide a simple, acceptable specimen collection technique that ensures consistent sample quality and produces accurate and precise blood Phe results which are interchangeable with plasma Phe.

Evaluation of volumetric blood collection devices for the measurement of phenylalanine and tyrosine to monitor patients with phenylketonuria.

BACKGROUND: Measurement of dried blood spot (DBS) phenylalanine (Phe) is central to the monitoring of patients with phenylketonuria. However, the volume and hematocrit (Hct) of the blood applied to conventional DBS cards significantly affects analytical results. Volumetric blood collection devices are reported to be more accurate, precise and less prone to Hct effects. METHODS: Accuracy, imprecision, effect of blood volume and Hct were evaluated for measurement of Phe and tyrosine using three volumetric devices and compared with the conventional PerkinElmer-226 filter-paper collection devices. i.e. conventional DBS cards. Applicability for use in a clinical laboratory was assessed qualitatively. RESULTS: Blood volume did not impact on the performance of the volumetric devices; however, significant biases were observed with the conventional DBS card. A higher Hct introduced unacceptable bias for Neoteryx-Mitra and conventional DBS card. All devices had a mean relative standard deviation (RSD) ≤ 4.1 %, except for the Neoteryx-Mitra (≤ 6.2 %). Relative to liquid blood, the mean biases of Phe for the various devices were -5.1 (HemaXis-DB10), -7.8 (Capitainer-qDBS), -12.0 (Neoteryx-Mitra) and -32.6 % (conventional DBS card). CONCLUSIONS: Introducing volumetric collection devices will overcome the significant pre-analytical issues associated with conventional DBS collection and improve the biochemical monitoring of patients with PKU.

Automation of tacrolimus measurement on volumetric absorptive microsampling devices by tandem mass spectrometry.

Aims: An automated method for the measurement of blood tacrolimus on volumetric absorptive microsampling (VAMS) devices was developed. Materials & methods: VAMS devices prepared by the automated method were compared with those prepared by the existing manual method (n = 284; mean concentration: 8.0 µg/l; range: 0.6-18.1). Results: The performance of both methods was comparable. Passing-Bablok regression demonstrated an acceptable correlation (y = -0.449 + 1.06x). Bland-Altman analysis demonstrated acceptable agreement (mean bias: -0.007 µg/l; standard deviation: 1.536). Automation reduced operator touch time by 40 min (48-sample batch). Conclusion: Automated preparation of VAMS devices reduced touch time and improved process consistency, facilitating high-throughput testing and transformation of existing laboratory workflows. Automation did not improve precision for VAMS devices but did so for liquid blood samples.

After a kidney transplant, many patients take a drug called tacrolimus to help prevent their new kidney from being rejected. Blood levels of tacrolimus are checked regularly to ensure each patient is receiving the right dose. This means regular visits to the hospital for blood tests, which can be inconvenient and time-consuming for the patient. Microsampling devices are now available that would enable patients to collect blood from a finger prick sample, at home, and post it back to the lab for testing. However, to date, access to home sampling is limited because measuring tacrolimus from blood collected on a microsampling device relies on a manual laboratory process that is difficult to do and takes a long time. Measurement of tacrolimus from blood collected on a microsampling device can be successfully automated with a Gerstel MPS robot. The robot extracts the tacrolimus from the blood on the microsampling device and injects the resulting sample into a mass spectrometer for measurement. Two sets of microsamples were prepared. One set of samples was extracted by the robot and one set of VAMS samples was extracted manually. Tacrolimus was measured by mass spectrometry for both sets of samples and the results compared well. The automated method requires less operator input than the manual method, which will make it easier to measure large numbers of microsamples quickly and safely, increasing the number of patients who can benefit from the advantages of remote sampling.