An Overview of Heparin Monitoring with the Anti-Xa Assay

Heparin remains a critical therapy in hospitalized patients requiring anticoagulation. Unfractionated heparin (UFH) mediates its therapeutic effect by binding to antithrombin (AT) and inhibiting thrombin and FXa, as well as other serine proteases. Because of its complex pharmacokinetics, monitoring UFH therapy is required, which is usually achieved with either the activated partial thromboplastin time (APTT) or the anti-factor Xa (anti-Xa) assay. Low molecular weight heparin (LMWH) is fast replacing UFH, as it has a more predictable response, negating the need for routine monitoring in most cases. When required, the anti-Xa assay is used for monitoring of LMWH. The APTT has many notable limitations when used for heparin therapeutic monitoring, including biologic, preanalytical, and analytical issues. With its increasing availability, the anti-Xa assay is appealing as it is less affected by patient factors (e.g., acute-phase reactants, lupus anticoagulants, consumptive coagulopathies), known to interfere with the APTT. The anti-Xa assay has shown additional benefits, such as faster time to achieve therapeutic levels, more consistent therapeutic levels, less dose adjustments, and, overall, less tests performed during therapy. However, poor interlaboratory agreement has been observed among anti-Xa reagents, highlighting that further work needs to be done to standardize this assay for use in patient heparin monitoring.

A Sticky Situation: Variable Agreement Between Platelet Function Tests Used to Assess Anti-platelet Therapy Response

Background

Platelet function testing to monitor antiplatelet therapy is important for reducing thromboembolic complications, yet variability across testing methods remains challenging. Here we evaluated the agreement of four different testing platforms used to monitor antiplatelet effects of aspirin (ASA) or P2Y₁₂ inhibitors (P2Y12-I).

Methods

Blood and urine specimens from 20 patients receiving dual antiplatelet therapy were analyzed by light transmission aggregometry (LTA), whole blood aggregometry (WBA), VerifyNow PRUTest and AspirinWorks. Result interpretation based on pre-defined cutoff values was used to calculate raw agreement indices, and Pearson's correlation coefficient determined using individual units of measure.

Results

Agreement between LTA and WBA for P2Y12-I-response was 60% (r = 0.65, high-dose ADP; r = 0.75, low-dose ADP). VerifyNow agreed with LTA in 75% (r = 0.86, high-dose ADP; r = 0.75, low-dose ADP) and WBA in 55% (r = 0.57) of cases. Agreement between LTA and WBA for ASA-response was 45% (r = 0.09, high-dose collagen WBA; r = 0.19, low-dose collagen WBA). AspirinWorks agreed with LTA in 60% (r = 0.32) and WBA in 35% (r = 0.02, high-dose collagen WBA; r = 0.08, low-dose collagen WBA) of cases.

Conclusions

Overall agreement varied from 35 to 75%. LTA and VerifyNow demonstrated the highest agreement for P2Y12-I-response, followed by moderate agreement between LTA and WBA. LTA and AspirinWorks showed moderate agreement for aspirin response, while WBA showed the weakest agreement with both LTA and AspirinWorks. The results from this study support the continued use of LTA for monitoring dual antiplatelet therapy, with VerifyNow as an appropriate alternative for P2Y12-I-response. Integration of results obtained from these varied testing platforms with patient outcomes remains paramount for future studies.

Characterization of Ebola convalescent plasma donor immune response and psoralen treated plasma in the United States

BACKGROUND

In 2014, passive immunization by transfusion of Ebola convalescent plasma (ECP) was considered for treating patients with acute Ebola virus disease (EVD). Early Ebola virus (EBOV) seroconversion confers a survival advantage in natural infection, hence transfusion of ECP plasma with high levels of neutralizing EBOV antibodies is a potential passive immune therapy. Techniques to reduce the risk of other transfusion-transmitted infections (TTIs) are warranted as recent ECP survivors are ineligible as routine blood donors. As part of an ongoing clinical trial to evaluate the safety and effectiveness of ECP, the impact of amotosalen/UVA pathogen reduction technology (PRT) on EBOV antibody characteristics was examined.

STUDY DESIGN AND METHODS

Serum and plasma samples were collected from EVD-recovered subjects at multiple timepoints and evaluated by ELISA for antibodies to recombinant EBOV glycoprotein (GP) and irradiated whole EBOV antigen, as well as for EBOV microneutralization, classic plaque reduction neutralization test (PRNT) and EBOV pseudovirion neutralization assay (PsVNA) activity.

RESULTS

Six subjects donated 40 individual ECP units. Substantial antibody titers and neutralizing activity results were demonstrated but were generally lower for the ACD plasma samples compared to the serum samples. Anti-EBOV titers by all assays remained essentially unchanged after PRT.

CONCLUSION

Treatment of ECP with PRT to reduce the risk of TTI did not significantly reduce EBOV IgG antibody titers or neutralizing activity. Although ECP was used in the treatment of repatriated patients, no PRT units from this study were transfused to EVD patients. This inventory of PRT-treated ECP is currently available for future clinical evaluation.

Identification of a recurrent pattern of false-positivity by Luminex HLA MHC class I single antigen bead testing

Previously, a distinct MHC class II Luminex-single antigen bead (SAB) pattern was described and attributed to antibodies targeting denatured antigens. In this study, we describe a distinct MHC class I reactivity pattern observed in 1.8% (105/5992) of samples resulted in 2017. The pattern displays reactivity to the following Luminex-SABs: HLA-A*33:03, A*36:01, A*80:01, B*54:01, B*53:01, C*06:02, C*07:02, C*18:02, C*14:02, C*03:03, C*03:04, and C*15:02. This pattern was identified in patients with no sensitization history, negative FlowPRA results, and antibody to self-antigen(s). Epitope analysis failed to reveal a common determinant(s) to explain this pattern of reactivity. Additionally, we found this pattern to be prevalent in female patients (62%) and also those with systemic lupus erythematosus (62%). Given these findings, we speculate this pattern likely represents false-positive reactivity, possibly due to antibody targeting denatured antigens or a specific peptide, molecular mimicry, autoimmunity, or a combination thereof.

How do I … implement diagnostic management teams in transfusion medicine?

Diagnostic management teams (DMTs) were conceptualized approximately twenty years ago in response to increasing subspecialization in medicine. DMTs are a collaboration between diagnostic experts and clinicians that aim to improve accurate and timely diagnosis and treatment of disease. Diagnostic experts provide their expertise in the increasingly complex realm of laboratory testing and interpretation of those test results to guide appropriate test utilization for individual patients. Not only can this approach improve patient care and safety, but DMTs also decrease healthcare costs by reducing unnecessary testing and potential diagnostic errors. Following the DMT construct and principles along with the 2015 National Academy of Medicine recommendations, our transfusion medicine (TM) service streamlined the workup and management of platelet refractory (PR) patients by developing and implementing a formal PR laboratory consult. The goals of this DMT and consult are to improve diagnostic management of PR patients and to decrease delays in providing these patients with appropriate and compatible platelet units. A comprehensive interpretation of test results is directly uploaded to the patient's electronic medical record (EMR), which is associated with a CPT code allowing for compensation for the PR evaluation. Herein we describe the development of and experience with the DMT since its implementation.

A Sticky Situation: Poor Correlation Between Platelet Inhibition Assays

Monitoring antiplatelet therapy is critical in patients undergoing cardiac or neurologic stent placement to ensure adequate platelet suppression and prevent thrombosis. Light transmission aggregometry (LTA) using platelet-rich plasma (PRP) is the gold standard to assess platelet responsiveness to aspirin (ASA) and P2Y12 inhibitors (P2Y12-I). Other platforms for antiplatelet therapy monitoring employ whole-blood aggregometry (WBA) via electrical impedance (EA) or ELISA-based detection of urinary metabolites. The goal of this study was to evaluate the concordance of four antiplatelet therapy monitoring platforms. Blood and urine samples from 20 patients receiving antiplatelet therapy prior to neurologic stent placement were prospectively collected. Blood samples were analyzed by LTA using PRP on the Helena AggRAM and by WBA-EA using the Chrono-log Lumi-aggregometer to assess ASA and P2Y12-I response. For the LTA, the maximum amplitude (MA) of the platelet aggregation curve was determined using high-dose (HD) and low-dose (LD) ADP agonist for measuring P2Y12-I response and with arachidonic acid (AA) for ASA response. For the WBA, resistance (ohms) was measured with ADP for P2Y12-I response and with HD and LD collagen for ASA response. Whole-blood samples were also analyzed with the VerifyNow PRUTest to assess platelet response to P2Y12-I. Urine samples were analyzed with AspirinWorks. Concordance of antiplatelet response based on predefined cutoff values (eg, concordant = optimal/optimal, discordant = optimal/suboptimal), as well as correlation between actual units of measure, was determined. When comparing P2Y12-I response determined by LTA and WBA, overall concordance was 55% (correlation was r = 0.75 and 0.65 for HD and LD ADP, respectively). VerifyNow was concordant with LTA in 70% (r = 0.86 and 0.75 for HD and LD ADP, respectively) and with WBA in 45% (r = 0.57) of samples. When comparing ASA response between LTA and WBA, overall concordance was 70% (r = 0.09 and 0.19 for HD and LD collagen, respectively). AspirinWorks was concordant with LTA and WBA in 60% of samples (r = 0.32 for LTA; r = 0.08 LD collagen, r = 0.02 for HD collagen on WBA). In summary, concordance varied between 45% and 70% on the four antiplatelet therapy monitoring platforms. Testing of P2Y12 inhibition showed the best correlation between LTA HD ADP and VerifyNow (r = 0.86). When comparing WBA vs LTA, the strongest correlation was between LD ADP LTA and WBA (r = 0.75). Correlation with ASA inhibition was overall poor between the various assays (r ranging between 0.02 and 0.58). The results from this correlation study support the continued use of LTA with HD and LD ADP and AA; other testing methods can be used as adjuncts when LTA proves difficult to interpret.

Diagnostic Management Team: Platelet Refractory Algorithm and Consult

Platelet refractoriness occurs when a patient fails to achieve appropriate platelet count increment following platelet (plt) transfusion. Approximately 30% to 40% of cases are due to human leukocyte antigen (HLA) alloantibodies and 2% to 10% are due to human platelet antigen (HPA) alloantibodies, which can be detected by various assays. The results of these tests are then used to guide selection of appropriate units, such as crossmatch-compatible (XM) and HLA-matched (HLAm) plts. Given the various steps and tests involved in diagnosing and managing plt refractoriness, clinicians may order unnecessary tests resulting in delays in patient care. In April 2018, our institution formed a diagnostic management team (DMT) to establish an algorithmic approach to testing, diagnosing, and transfusing plt refractory patients.

We performed a retrospective review of clinician requests for XM or HLAm plts over a 9-month period prior to the launch of the DMT and compared that to requests placed during the initial 9-month period following the launch of the DMT. We collected the date transfusion medicine (TM) service was notified and the time to complete the following: 1-hour corrected count increment (CCI), ELISA indirect antibody screen, HLA flow cytometry PRA screen, HLA single antigen bead (SAB), and obtain the first XM or HLAm plt unit for the patient. There were 12 and 20 patients evaluated for plt refractoriness pre- and post-DMT, respectively. The median time to complete the ELISA indirect antibody screen was 2 days pre- and post-DMT, respectively. Eight percent of patients never had an ELISA screen performed pre-DMT. All patients meeting plt refractory criteria had ELISA screens performed post-DMT. Median time to complete HLA testing decreased from 4 days to 2 days and all patients who had positive FlowPRAs had subsequent SABs performed post-DMT compared to only 40% of patients pre-DMT. The median times to obtain the first XM unit were 6 and 5 days and the first HLAm unit was 5 and 6 days pre- and post-DMT, respectively. One patient had a CCI above 7,500 and four patients had negative screening ELISA and FlowPRA tests after the launch of the DMT. No further testing was performed and they continued to receive ABO-compatible (ABOc) plt units. Since the launch of the DMT, there is improved consistency regarding what tests are performed and standardization of the sequence in which the appropriate tests are ordered. We have reduced unnecessary testing by identifying patients with nonimmune-mediated etiologies of plt refractoriness. Divergence from the algorithm is the main issue encountered during the initial 9-month period following implementation of the DMT. This divergence stemmed from lack of consistent education about plt refractoriness and the DMT. We believe better adherence to the algorithm will lead to decreased product acquisition time and further improve efficiency.

Verification of a Novel Multiplex PCR Respiratory Virus Panel in a US Biocontainment Unit

Emerging infectious diseases carry unique logistical, financial, and clinical ramifications. Rapid diagnostic testing methods can alleviate some of these challenges by providing definitive diagnoses earlier in the clinical course, leading to appropriate targeted therapy, cost savings, and improved patient outcomes. The BioFire FilmArray Respiratory Panel 2 plus (RP2plus; bioMérieux, Marcy l’Etoile, France) is a multiplexed nucleic acid test for detection of Middle East respiratory syndrome coronavirus (MERS-CoV) and 14 common viral and 4 bacterial respiratory pathogens in nasopharyngeal swabs obtained from those meeting MERS-CoV epidemiological criteria. The aim of this study was to verify the FilmArray RP2plus for use in our biocontainment unit. Of note, the RP2plus is FDA approved but not currently available for sale in the United States. Eight patient samples were tested with known results (GenMark Respiratory Virus Panel [RVP] or Cepheid Xpert Flu/RSV). We had concordant results between the platforms for samples containing influenza A, respiratory syncytial virus (RSV), parainfluenza virus 2, rhinovirus, and a negative sample. We evaluated two influenza B samples from two different patients. The FilmArray RP2plus did not detect influenza B in one of the patient samples. The sample was exhausted and repeat testing could not be performed. A second rhinovirus sample was not detected by the RP2plus, but Coronavirus 229E was detected in this sample, a virus not detected by the RVP. The sample was repeated and again did not detect rhinovirus. Further investigation into this discrepancy revealed that rhinovirus was originally detected by RVP at a signal of 34.4 nA (repeat of 46.9 nA). The concordant rhinovirus sample had a signal of 226.7 nA by RVP, which was much higher than the discrepant sample. Because of the low signal by RVP in the discrepant sample, perhaps the viral load was below the limit of detection of the RP2plus. All other quality control sample pools passed verification testing, including day-to-day and operator variance. It is not uncommon for a person under investigation (PUI) for a highly communicable disease to be evaluated in our facility. The performance of the RP2plus test on clinical samples showed acceptable concordance with our current means of testing for respiratory pathogens. The RP2plus will eliminate challenges implicated in storing and transporting specimens to an off-site lab, facilitate quicker turnaround time, and streamline the often cumbersome, complex protocols and practices required to work up a serious communicable disease.

Caring for patients with Ebola virus disease: Are U.S. biocontainment centers ready for the next outbreak?

The 2014 West African Ebola virus disease (EVD) outbreak is the largest and deadliest EVD epidemic to date, resulting in fivefold more cases than all other outbreaks combined. This outbreak was particularly devastating to healthcare workers in West Africa and resulted in several EVD patients being medically evacuated for treatment in the U.S. and Europe. Governmental agencies provide recommendations for triaging and testing patients with EVD, however best laboratory practices are still unknown and are very resource dependent.

Challenges in the treatment and prevention of delayed hemolytic transfusion reactions with hyperhemolysis in sickle cell disease patients

BACKGROUND

Delayed hemolytic transfusion reactions (DHTRs) are serious complications of RBC transfusion that can occur in previously alloimmunized patients. Patients who require episodic transfusions during heightened inflammatory states, such as patients with sickle cell disease (SCD), are particularly prone to alloimmunization and developing DHTRs with hyperhemolysis. While efforts to mitigate these hemolytic episodes via immunosuppressive drugs can be employed, the relative efficacy of various treatment options remains incompletely understood.

CASE REPORTS

In this study, we explored five patients with SCD and multiple RBC alloantibodies who received various forms of immunosuppressive therapy in an attempt to prevent or treat severe DHTRs.

RESULTS

The clinical course for these five patients provides insight into the difficulty of effectively treating and preventing DHTRs in patients with SCD with currently available immunosuppressive therapies.

CONCLUSION

Based on our experience, and the current literature, it is difficult to predict the potential impact of various immunosuppressive therapies when seeking to prevent or treat DHTRs. Future mechanistic studies are needed to identify the optimal treatment options for DHTRs in the presence or absence of distinct alloantibodies in patients with SCD.

Stability of anti-A blood group titers among blood group B renal transplant candidates

BACKGROUND

As deceased donor kidney allocation is based in part on blood type compatibility, group B candidates are disadvantaged due to their disproportionate representation on the wait list compared to the group B donor pool. To mitigate this discrepancy, group B candidates can receive group A₂ or A₂ B donor kidneys if their anti-A titers are below a predetermined cutoff. Currently, eligibility is reverified quarterly to UNet based on individual center protocols, which can vary due to a lack of set guidelines for monitoring ABO titers in these patients. Our goal was to assess the stability of anti-A titers in blood group B renal transplant candidates over time to provide data that could aid in the development of standardized ABO titer protocols.

STUDY DESIGN AND METHODS

Titers performed between January 2011 and December 2015 were assessed for 191 group B patients with two or more documented titers.

RESULTS

Fifty patients (26%) were ineligible, as the first titer exceeded the cutoff of 8. Of the remaining 141 patients, 19 (13%) became ineligible as the second titer exceeded 8. Thirty-nine patients (28%) had no change in titer between samples, while 71 (50%) had a titer change that never exceeded 8. Only 12 patients (8.5% of total) experienced a titer change that affected eligibility after the second test.

CONCLUSION

Although patients experience some variability in anti-A titers over time, in most cases, stability did not affect candidate eligibility. Our results indicate that regular testing beyond the second titer may be unnecessary and represent test overutilization.

(F)Utility of the physical crossmatch for living donor evaluations in the age of the virtual crossmatch

Flow cytometric crossmatches (FCXM) are routinely performed to support living-donor renal transplantation. While long a laboratory mainstay, a physical crossmatch is costly, time consuming, and frequently poses interpretative conundrums with both false-positive and false- negative results. Given the increased utilization of the virtual crossmatch (vXM) in the deceased donor setting, our aim was to assess its utility in living donor evaluations. We reviewed 100 living donor FCXMs and retrospectively performed a vXM for each pair. Seventy-five (75) cases were concordant, (i.e., FCXM-/vXM- or FCXM+/vXM+) while 25 cases were discordant; Five were vXM+/FCXM- and 20 were FCXM+/vXM-. Since donor-specific antibodies (DSA) were not detected in the 20 FCXM+/vXM- cases, these were interpreted as false-positive, i.e., due to non-HLA antibodies. Importantly, none of these patients, when transplanted across a positive FCXM, experienced early antibody mediated rejection or subsequently developed HLA DSA. These data reveal that, for the vast majority of living donor evaluations, a vXM is an acceptable vetting procedure.

Transfusion-transmitted malaria masquerading as sickle cell crisis with multisystem organ failure

BACKGROUND

Fever accompanying vaso-occlusive crisis is a common presentation in patients with sickle cell disease (SCD) and carries a broad differential diagnosis. Here, we report a case of transfusion-transmitted malaria in a patient with SCD presenting with acute vaso-occlusive crisis and rapidly decompensating to multisystem organ failure (MSOF).

CASE REPORT

An 18-year-old African American male with SCD was admitted after multiple days of fever and severe generalized body pain. He received monthly blood transfusions as stroke prophylaxis. A source of infection was not readily identified, but treatment was initiated with continuous intravenous fluids and empiric antibiotics. The patient developed acute renal failure, acute hypoxic respiratory failure, and shock. He underwent red blood cell (RBC) exchange transfusion followed by therapeutic plasma exchange and continuous veno-venous hemodialysis. A manual peripheral blood smear revealed intraerythrocytic inclusions suggestive of Plasmodium, and molecular studies confirmed Plasmodium falciparum infection. Intravenous artesunate was given daily for 1 week. A look-back investigation involving two hospitals, multiple blood suppliers, and state and federal public health departments identified the source of malaria as a unit of RBCs transfused 2 weeks prior to admission.

CONCLUSIONS

Clinical suspicion for transfusion-related adverse events, including hemolytic transfusion reactions and transfusion-transmitted infections, should be high in typically and atypically immunocompromised patient populations (like SCD), especially those on chronic transfusion protocols. Manual blood smear review aids in the evaluation of patients with SCD presenting with severe vaso-occlusive crisis and MSOF and can alert clinicians to the need for initiating aggressive therapy like RBC exchange and artesunate therapy.

The evolutionary history of the genus Acanthamoeba and the identification of eight new 18S rRNA gene sequence types

The 18S rRNA gene (Rns) phylogeny of Acanthamoeba is being investigated as a basis for improvements in the nomenclature and taxonomy of the genus. We previously analyzed Rns sequences from 18 isolates from morphological groups 2 and 3 and found that they fell into four distinct evolutionary lineages we called sequence types T1-T4. Here, we analyzed sequences from 53 isolates representing 16 species and including 35 new strains. Eight additional lineages (sequence types T5-T12) were identified. Four of the 12 sequence types included strains from more than one nominal species. Thus, sequence types could be equated with species in some cases or with complexes of closely related species in others. The largest complex, sequence type T4, which contained six closely related nominal species, included 24 of 25 keratitis isolates. Rns sequence variation was insufficient for full phylogenetic resolution of branching orders within this complex, but the mixing of species observed at terminal nodes confirmed that traditional classification of isolates has been inconsistent. One solution to this problem would be to equate sequence types and single species. Alternatively, additional molecular information will be required to reliably differentiate species within the complexes. Three sequence types of morphological group 1 species represented the earliest divergence in the history of the genus and, based on their genetic distinctiveness, are candidates for reclassification as one or more novel genera.

Long-term results of lower-extremity venous injuries

OBJECTIVES

To compare the long-term venous function of ligated, simple, and complex repairs and to assess long-term patency in repaired veins.

DESIGN

A cohort study of patients with lower-extremity venous injuries treated during a 7-year period.

SETTING

A level I urban trauma center.

PATIENTS

Twenty-one of the 79 patients with a history of lower-extremity venous injury identified via the trauma registry consented to outpatient evaluation.

INTERVENTION

Participating patients underwent a through vascular examination that included color flow duplex venous imaging and air plethysmographic assessment.

MAIN OUTCOME MEASURES

The patency of venous repairs, the incidence of chronic deep venous thrombosis, and evidence of chronic venous insufficiency.

RESULTS

The venous injuries included 5 iliac, 10 femoral, and 6 popliteal. Six of these injuries were ligated, 11 injuries were simply repaired (lateral venorrhaphy or end-to-end), and 4 were repaired with complex interposition grafts. All repairs were patent, with no evidence of deep venous thrombosis by color flow duplex venous imaging. Seventeen of the 21 patients had symptoms, color flow duplex venous imaging findings, and air plethysmographic data consistent with chronic venous insufficiency, including significant mean differences (P < .03) in outflow fraction, outflow fraction with compression, venous filling index, and residual volume fraction, when compared with the uninjured extremity. The most profound changes followed complex repairs and perioperative fasciotomies.

CONCLUSIONS

While the long-term patency of venous repairs is excellent, most patients demonstrate evidence of chronic venous insufficiency after either ligation or repair. Complex venous repairs and fasciotomy are associated with the most severe functional changes.

Unraveling bleeding problems in midlife women

Menopause often is accompanied by abnormal uterine bleeding-mostly due to anovulation and correctable with hormone therapy. However, all patients should be fully evaluated for gynecologic and nongynecologic causes of bleeding.

Homogeneous apoenzyme reactivation immunoassay for thyroxin-binding globulin in serum

In this automated apoenzyme reactivation immunoassay system (Ames Optimate) for thyroxin-binding globulin (TBG), the sample and N6-aminohexylflavin adenine dinucleotide-labeled TBG react sequentially with antiserum. Then apoglucose oxidase is added to combine with the free fraction and generate glucose oxidase activity, which is measured colorimetrically. The assay requires 100 microL of sample and covers the clinically significant range for TBG (less than 2.5 to 55 mg/L). The first result is obtained in 16 min; assay of 29 samples and their blanks is completed in less than 1 h. The lower limit of detection is about 2.5 mg/L. Between-assay CVs (n = 9) were less than 9%, within-assay CVs (n = 5) were less than 6%, and analytical recovery of TBG was 103-112%. Reagents are stable at 4 degrees C for at least five months. Results by this method for serum TBG (y) compared well with those determined by radioimmunoassay (x): y = 1.029x--0.352 (r = 0.990, n = 49, Syx = 1.165 mg/L). In addition, with 39 other sera the ratio of total thyroxin (by RIA) to TBG compared well with free thyroxin measured by equilibrium dialysis (r = 0.930) and the free thyroxin index (r = 0.970).

Homogeneous apoenzyme reactivation immunoassay for thyroxin-binding globulin in serum

In this automated apoenzyme reactivation immunoassay system (Ames Optimate) for thyroxin-binding globulin (TBG), the sample and N6-aminohexylflavin adenine dinucleotide-labeled TBG react sequentially with antiserum. Then apoglucose oxidase is added to combine with the free fraction and generate glucose oxidase activity, which is measured colorimetrically. The assay requires 100 microL of sample and covers the clinically significant range for TBG (less than 2.5 to 55 mg/L). The first result is obtained in 16 min; assay of 29 samples and their blanks is completed in less than 1 h. The lower limit of detection is about 2.5 mg/L. Between-assay CVs (n = 9) were less than 9%, within-assay CVs (n = 5) were less than 6%, and analytical recovery of TBG was 103-112%. Reagents are stable at 4 degrees C for at least five months. Results by this method for serum TBG (y) compared well with those determined by radioimmunoassay (x): y = 1.029x--0.352 (r = 0.990, n = 49, Syx = 1.165 mg/L). In addition, with 39 other sera the ratio of total thyroxin (by RIA) to TBG compared well with free thyroxin measured by equilibrium dialysis (r = 0.930) and the free thyroxin index (r = 0.970).

Coupling aminohexyl-FAD to proteins with dimethyladipimidate

We used an efficient method having general applicability to couple N6-aminohexyl-flavin adenine dinucleotide (AHFAD) to several proteins for use in an apoenzyme reactivation immunoassay system (ARIS). AHFAD is first activated with 40-fold molar excess of dimethyladipimidate, excess imidate is removed rapidly by gel filtration, the activated product is incubated with the protein, and the conjugate formed is purified. This labeling technique permits incorporation of a controlled amount of amino-label into a protein, and eliminates the possibility of self-crosslinking, which would reduce the immunoreactivity of the conjugate. Here we demonstrate the utility of such a conjugate in a totally automated ARIS assay for thyroxin-binding globulin (TBG). After a competitive protein-binding reaction, apoglucose oxidase is added to combine with free TBG-AHFAD conjugate and produce active glucose oxidase, which is measured colorimetrically in a peroxidase-linked reaction. The assay covers the clinically significant range for TBG from 0 to 60 mg/L and has a throughput of 60 reactions in 75 min. Comparison with an RIA method (x) by regression analysis yielded the equation y = 0.890x + 1.217 (r = 0.975, n = 47, Syx = 1.906 mg/L).

Coupling aminohexyl-FAD to proteins with dimethyladipimidate

We used an efficient method having general applicability to couple N6-aminohexyl-flavin adenine dinucleotide (AHFAD) to several proteins for use in an apoenzyme reactivation immunoassay system (ARIS). AHFAD is first activated with 40-fold molar excess of dimethyladipimidate, excess imidate is removed rapidly by gel filtration, the activated product is incubated with the protein, and the conjugate formed is purified. This labeling technique permits incorporation of a controlled amount of amino-label into a protein, and eliminates the possibility of self-crosslinking, which would reduce the immunoreactivity of the conjugate. Here we demonstrate the utility of such a conjugate in a totally automated ARIS assay for thyroxin-binding globulin (TBG). After a competitive protein-binding reaction, apoglucose oxidase is added to combine with free TBG-AHFAD conjugate and produce active glucose oxidase, which is measured colorimetrically in a peroxidase-linked reaction. The assay covers the clinically significant range for TBG from 0 to 60 mg/L and has a throughput of 60 reactions in 75 min. Comparison with an RIA method (x) by regression analysis yielded the equation y = 0.890x + 1.217 (r = 0.975, n = 47, Syx = 1.906 mg/L).