Christopher D. Koch

Precision and Reliability of 5 Platelet Function Tests in Healthy Volunteers and Donors on Daily Antiplatelet Agent Therapy

BACKGROUNDnAnticoagulation protocols used during mechanical circulatory support call for titration of antiplatelet agents. We compared the precision and reliability of 5 platelet function tests in healthy volunteers and donors on daily antiplatelet therapy to distinguish their efficacy for titrating antiplatelet therapy.nnnMETHODSnWe assessed arachidonic acid-induced platelet function by light transmission aggregometry (LTA), Multiplate impedance aggregometry, VerifyNow, and platelet mapping by thromboelastography (TEG PM). We assessed ADP-induced platelet function by the same methods and flow cytometry. Forty healthy volunteers and 10-13 volunteers on daily aspirin and/or clopidogrel therapy were evaluated. We compared tests for intraassay precision, interassay precision (samples from 2 separate blood draws), and reliability coefficient.nnnRESULTSnFor arachidonic acid-induced platelet aggregation in healthy volunteers, intra- and interassay CVs were ≤ 10% for all methods. Intra- and interassay precision among donors on daily aspirin was ≤ 30% for all methods except LTA (38% interassay CV) and TEG PM (95% intraassay and 104% interassay CV). For ADP-induced platelet function, intra- and interassay precision was ≤ 10% and ≤ 30% for all methods. Only Multiplate demonstrated moderate or greater (R > 0.40) reliability coefficients for arachidonic acid-induced platelet function among all subjects. All methods of ADP-induced platelet function, except TEG PM, demonstrated substantial or greater (R > 0.60) reliability among all subjects.nnnCONCLUSIONSnTEG PM is least suited to monitor effects of antiplatelet agents. Multiplate impedance aggregometry was the only method to demonstrate an acceptable reliability coefficient among healthy volunteers and donors on both aspirin and clopidogrel therapy.

BD rapid serum tubes reduce false positive plasma troponin T results on the Roche Cobas e411 analyzer

OBJECTIVESnIn an attempt to reduce false positive results and improve turnaround time, we investigated the BD Rapid Serum Tube as an alternate sample type to lithium heparin plasma for Roche Troponin T analysis on the Roche Cobas e411 analyzer.nnnDESIGN AND METHODSnBD Plasma Separator Tubes (PST) and Rapid Serum Tubes (RST) were collected in tandem from Emergency Department patients who had clinical orders for Troponin T over a 1 month period.nnnRESULTSnRST and PST samples yielded analytically and clinically concordant Troponin T results on the Roche Cobas e411. Rare false positive results in lithium heparin samples were not observed with rapid clot serum tubes.nnnCONCLUSIONSnRST samples are appropriate for stat Troponin T analysis, and appear to reduce the incidence of rare false positive Troponin T results obtained with lithium heparin samples.

Local verification and assignment of mean normal prothrombin time and International Sensitivity Index values across various instruments: recent experience and outcome from North America.

Warfarin dosing relies on accurate measurements of international normalized ratio (INR), which is calculated from the prothrombin time (PT), International Sensitivity Index international sensitivity index (ISI) of the thromboplastin, and the geometric mean of normal PT (MNPT). However, ISI assignments of certain reagent/instrument combinations are frequently unavailable, especially when the reagent and instrument are not from the same manufacturer. The effort to be in compliance with widely endorsed Clinical and Laboratory Standards Institute (CLSI) guidelines by locally verifying or assigning an ISI to an unsupported reagent/instrument combination is further hindered by the lack of US Food and Drug Administration (FDA)-approved certified plasmas designated for a particular reagent/instrument combination. The objectives of the study include development of a process to verify/assign ISI and MNPT of a single thromboplastin reagent from one manufacturer across multiple instruments including several from another manufacturer and across several campuses of a single institution, the Mayo Clinic. In this study, RecombiPlasTin 2G (R2G), was evaluated on the ACL TOP 700 (IL), STA-R Evolution, STA Compact, and STA Satellite. Random normal donor samples (n = 25) were used to verify/assign MNPT. A subset of the normal donors (n = 8) and 13 warfarin pools (INR range: 1.3-3.9), created from stable warfarin patient plasma, were used for ISI verification/assignment. The manufacturers assigned ISI was first verified on the ACL TOP 700 (reference method), then assigned on three unsupported instruments using orthogonal regression analysis. The MNPT and manufacturer assigned ISI (11.0, 0.95) were verified on the ACL TOP 700 and subsequently assigned on the STA-R Evolution (11.6, 1.04); STA Compact (11.5, 1.02); and STA Satellite (10.9, 0.99). Linear correlations of the INR results from all the four instruments demonstrated an r2 > 0.99. This process provides a reproducible approach to assigning ISIs on unsupported reagent/instrument combinations. Our data also confirm that ISIs of the same PT reagent differ significantly on different instruments, thus confirming the requirement for evaluations and validation of ISIs for different reagent/instrument combinations.

Accuracy of whole blood glucose measurement when venous catheter blood samples are used on glucose meters.

BACKGROUNDnPrevious studies have found positive bias and frequent outliers when central venous catheter (CVC) whole blood is used to dose glucose meters. We designed a study to determine whether positive bias and outliers with CVC whole blood glucose samples are due to exogenous glucose contamination of CVC samples, inherent bias and imprecision of glucose meters, or properties of CVC whole blood that interfere with the function of some glucose meters.nnnMETHODSnWe studied the relationship between venous whole blood and venous plasma glucose drawn by venipuncture to CVC whole blood and CVC plasma glucose in 50 hospitalized patients. In 27 patients whole blood glucose was measured on both the Accu-Chek Inform (Roche Diagnostics, Indianapolis, IN) and StatStrip (Nova Biomedical, Waltham, MA).nnnRESULTSnBy comparing CVC plasma to venous plasma glucose, we determined that contamination of CVC samples with exogenous glucose was uncommon. On the Inform meter outliers were approximately twice as common with CVC whole blood compared to venous whole blood. In 27 patients who had CVC whole blood analyzed by both Inform and StatStrip, outliers occurred approximately twice as often on the Inform compared to the StatStrip. Accounting for CVC samples contaminated with exogenous glucose, outliers on the StatStrip did not occur significantly more often using CVC whole blood compared to venous whole blood.nnnCONCLUSIONSnProperties unique to CVC whole blood differentially affect glucose meter bias and imprecision. Device selection is critical in practices that wish to use CVC whole blood to monitor glucose concentration in hospitalized patients.

Fresh and Citrated Whole-Blood Specimens Can Produce Different Thromboelastography Results in Patients on Extracorporeal Membrane Oxygenation

OBJECTIVESnTo compare thromboelastography (TEG) tracings obtained from fresh and citrated whole-blood samples in patients on extracorporeal membrane oxygenation (ECMO) or after cardiopulmonary bypass and in healthy volunteers.nnnMETHODSnSamples of fresh and citrated whole blood were analyzed for 25 patients and 4 healthy volunteers. Thromboelastography analysis was performed in both plain and heparinase cups.nnnRESULTSnIn 5 of 6 patients on ECMO, use of citrated samples resulted in apparent partial or complete heparin reversal. In TEG tracings from patients following cardiopulmonary bypass, there was a slight hypercoagulable appearance in the citrated sample. No differences were noted between fresh and citrated samples from healthy volunteers whose blood was spiked with heparin.nnnCONCLUSIONSnIn some patients on ECMO, use of samples collected in sodium citrate tubes for TEG analysis results in significant artifacts, which could lead to heparin overdosing in these patients.

Lipid emulsion solution: A novel cause of hemolysis in serum and plasma blood samples

OBJECTIVESnAfter several hemolyzed blood samples were received in the laboratory, we investigated lipid emulsion/TPN as a novel cause of hemolysis.nnnDESIGN AND METHODSnWhole blood was spiked with lipid emulsion and TPN.nnnRESULTSnHemolysis was proportional to the amount of lipid emulsion present in whole blood, with less hemolysis occurring in blood gas syringes compared to vacutainer tubes.nnnCONCLUSIONnCollection of specimens in blood gas syringes may prevent hemolysis in patients on lipid emulsion.

Analytical performance of three whole blood point-of-care lactate devices compared to plasma lactate comparison methods and a flow-injection mass spectrometry method

OBJECTIVESnPoint of care (POC) whole blood lactate testing may facilitate rapid detection of sepsis. We evaluated three POC methods against both plasma lactate comparison methods and a flow-injection mass spectrometric (MS) method.nnnDESIGN AND METHODSnNova StatStrip, Abbott i-STAT CG4+ and Radiometer ABL90 POC lactate methods were evaluated against the mean of Cobas Integra 400 and Vitros 350 plasma lactate. POC methods were also compared to a flow-injection mass spectrometric assay measuring lactate in ZnSO4-precipitated whole blood extracts. Intra- and inter-assay precision was determined using quality control material. Method comparison included specimens from normal donors at rest, after exertion, and after spiking with lactic acid.nnnRESULTSnIntra- and inter-assay coefficient of variation was <5% for i-STAT and ABL90; but ranged from 3.1-8.2% on two StatStrip meters. Mean (±SD) bias between POC and plasma lactate ranged from -0.2±0.9 (i-STAT and ABL90) to -0.4±1.2 (StatStrip) mmol/L. At concentrations >6mmol/L, all POC methods showed proportional negative bias compared to plasma methods; but this bias was not observed when compared to the MS method. Despite proportional negative bias, all POC methods demonstrated acceptable concordance (94-100%) with plasma lactate within the reference interval (<2.3mmol/L) and >4mmol/L, commonly used clinical cut-offs for detection of sepsis.nnnCONCLUSIONSnPOC lactate methods demonstrate acceptable concordance with plasma lactate across commonly used clinical cut-offs for detection of sepsis. Due to systematic negative bias at higher lactate concentrations, POC and plasma lactate should not be used interchangeably to monitor patients with elevated lactate concentrations.

Discordance between urine pH measured by dipstick and pH meter: Implications for methotrexate administration protocols

OBJECTIVESnTo minimize toxicity of high-dose methotrexate (MTX) therapy, urinary alkalinization with frequent monitoring of urine pH is required. Urine pH is usually assessed by fast and convenient dipstick methods. When urine color interferes with dipstick measurement, as occurs in patients receiving MTX, alternative methods such as pH meters are used. Nursing staff caring for patients on high-dose MTX reported that urine pH results from dipstick and pH analyzers were often clinically discordant. As a result urine pH by dipstick and pH meter were compared in patients on high-dose MTX therapy and patients with normal-colored urine samples.nnnDESIGN AND METHODSnWe measured urine pH by dipstick and pH meter in 116 urine samples from 4 patients receiving high-dose MTX therapy, and in 50 normal-colored urine samples from 50 patients not on MTX therapy.nnnRESULTSnIn patients on MTX therapy the mean (±standard deviation) bias between dipstick and pH meter urine pH was 0.7±0.4, compared to 0.4±0.3 in patients not on MTX. For patients on MTX clinical concordance between dipstick and pH meter urine results was poor around a clinical cut-off of pH 8.0. Of the 92 samples with a meter urine pH≤8.0, 72 had a discordant value by dipstick (pH>8).nnnCONCLUSIONSnUrine pH readings by dipstick and pH meter are not equivalent, and the bias between them is exacerbated in patients on MTX. Institutions with high-dose MTX therapy protocols should not alternate between dipstick and pH meter urine pH monitoring.

Agreement between whole blood and plasma sodium measurements in profound hyponatremia

OBJECTIVESnWe compared two different methods of whole blood sodium measurement to plasma sodium measurement using samples in the profoundly hyponatremic range (Na < 120 mmol/L).nnnDESIGN AND METHODSnWhole blood pools with a range of low sodium values were generated using combinations and dilutions of pooled electrolyte-balanced lithium heparin samples submitted for arterial blood gas analysis. Each pool was analyzed five times on a Radiometer 827 blood gas analyzer and iSTAT analyzer. Pools were centrifuged to produce plasma, which was analyzed five times on a Roche Cobas c501 chemistry analyzer. An additional 40 fresh (analyzed on day of collection) excess lithium heparin arterial blood gas samples from 36 patients were analyzed on the Radiometer 827, iSTAT, and Cobas c501 as described above. The setting was a tertiary referral center. Blood samples were collected from a combination of patients in the intensive care unit, operating theaters and emergency room.nnnRESULTSnAll methods demonstrated excellent precision, even in the profoundly hyponatremic measurement range (Na < 120 mmol/L using a plasma reference method). However, agreement between the methods varied with the degree of hyponatremia. In the profoundly hyponatremic range, Radiometer whole blood sodium values were nearly identical to plasma reference sodium, while iSTAT whole blood sodium showed a consistent positive bias relative to plasma sodium in this range.nnnCONCLUSIONnIf whole blood direct sodium measurements are compared with plasma sodium in profoundly hyponatremic patients consideration should be given to the use of Radiometer blood gas analyzers over iSTAT since the latter shows a positive bias relative to a plasma comparative method.

Analytical performance of three point of care methods for pleural fluid pH analysis

OBJECTIVEnThe performance of three point of care methods for pleural fluid pH analysis was compared to a currently validated blood gas analyzer.nnnDESIGN AND METHODSnAn ABL 725 (Radiometer America, Westlake, OH) was used as the reference method to evaluate three cartridge-based assays: ABL 90 FLEX (Radiometer), and i-STAT 1 (Abbott Point of Care, Abbott Park, IL) CG4+ and G3+ cartridges for pleural fluid pH analysis. Pooled residual pleural fluid samples and quality control material were analyzed to determine intra- and inter-assay precision. Method comparison was performed with spiked (n=40) and clinically-ordered (n=10) pleural fluid samples across the analytical measuring range.nnnRESULTSnAll methods demonstrated inter-assay CVs<0.1% at pH values of 7.1 and 7.6, and intra-assay CVs<0.3% at pH values of 7.2 and 7.7. Bland-Altman plots demonstrated clinically significant bias between ABL 725 and each cartridge-based method only at pH>7.6. For samples with pH<7.6 mean bias vs. ABL 725 was -0.01 for ABL 90 FLEX and 0.03 for i-STAT 1 CG4+ and G3+ cartridges. Clinical concordance using a decision limit of pH7.2 was 96-98% for the three methods.nnnCONCLUSIONSnAnalytical and clinical performance of the three cartridge-based methods was comparable to a validated blood gas analyzer for pleural fluid pH analysis. Cartridge-based pH methods offer the advantage of easier troubleshooting for clots and clogs as they use disposable electrodes. However cartridge-based methods are not currently FDA-approved for pleural fluid samples, such that additional validation would be required for this specimen type.