Serum anion gap revisited: a verified reference interval for contemporary use.

BACKGROUND: The anion gap (AG) is often used to evaluate acid-base disorders. The reference interval for normal AG is used to differentiate between raised (gap) or normal AG (non-gap) acidosis. Historically accepted AG values may not be valid with the evolution of modern analytical techniques and the reference interval requires revalidation. AIMS: To determine the reference interval for AG based on current laboratory techniques. METHODS: During a health-screening exercise, 284 participants with no major illnesses volunteered surplus blood for analysis. The samples were tested in an internationally accredited clinical laboratory. AG was calculated by [Na+ ] - [Cl- ] - [HCO3 - ] and AGK by [Na+ ] + [K+ ] - [Cl- ] - [HCO3 - ]. The reference interval was determined at 2.5th-97.5th percentiles. Analysis was further undertaken for a subcohort of 156 individuals with no suboptimal health indicators. RESULTS: Median age was 35 years, body mass index 23.4 kg/m2 and the glomerular filtration rate was 106 mL/min/1.73 m2 . Median AG was 13 mmol/L and the reference interval for normal AG is 10-18 mmol/L with a 99% level of confidence. Statistically significant differences in AG were detected for sex, race, obesity and serum albumin, but the difference was 1 mmol/L between subgroups. The reference interval was the same for the sub-cohort of 156 individuals. Median AGK was 17.7 mmol/L and reference interval was 14.6-22.5 mmol/L. CONCLUSIONS: The AG reference interval of 10-18 mmol/L is valid for laboratories with similar reference intervals for electrolytes. Lower values expected with current laboratory techniques were not observed. The median AG of 13 mmol/L may be used to differentiate gap acidosis, non-gap acidosis or mixed acid-base disorders.

Evaluation of the Abbott Alinity i Thyroid-Stimulating Hormone Receptor Antibody (TRAb) Chemiluminescent Microparticle Immunoassay (CMIA).

Background: We evaluated the performance of the Abbott thyroid-stimulating hormone receptor antibody chemiluminescent microparticle immunoassay (CMIA) on the Alinity i. Methods: Verification studies for precision, linearity, analytical measuring range, diagnostic cut offs for Graves' disease were performed. We compared the Abbott CMIA to an established TRAb assay (Roche electrochemiluminescence immunoassay). Method comparison analysis was performed between serum and plasma samples on the Abbott CMIA. Results: Repeatability (CV%) for TRAb were 4.07, 1.56, 0.71 and within-laboratory imprecision (CV%) were 4.07, 1.90, 0.71 at 3.0, 10.0, 30.0 IU/L of TRAb, respectively. Linearity and analytical measuring range were verified from 1.07-47.9 IU/L. The limit of the blank was 0 IU/L, limit of detection was 0.15 IU/L, and limit of quantification was 0.5 IU/L. Passing-Bablok analysis showed agreement between the two assays; Y-intercept = 0.787, slope = 1.04. Passing-Bablok analysis also showed agreement between the plasma and serum samples run on the Abbott CMIA; Y-intercept -0.17, slope = 0.97. Conclusions: The Abbott TRAb CMIA on the Alinity i performs within the manufacturer claims for assay precision, linearity, analytical measuring range, limit of blank, limit of detection, limit of quantitation and diagnostic cut offs for Graves' disease. Thus, the Abbott TRAb CMIA on the Alinity i is fit for clinical use.

210-Day Kinetics of Total, IgG, and Neutralizing Spike Antibodies across a Course of 3 Doses of BNT162b2 mRNA Vaccine.

Introduction: We tested the total spike antibody (S-Ab), IgG/IgM S-Ab, and neutralizing antibody (N-Ab) responses of COVID-19-naïve subjects from before their first BNT162b2 vaccination up to 210 days after boosting. Methods: We studied 136 COVID-19-naïve subjects who received three doses of the Pfizer mRNA vaccine (39 males, 97 females, mean age 43.8 ± 13.5 years) from January 2021 to May 2022. Serum was assessed for total S-Ab (Roche), IgG/M (Abbott), and N-Ab (Snibe). Results: Peak antibody levels were measured 20-30 days after each dose, with booster dosing eliciting significantly higher peak antibodies than the second dose: total S-Ab 2219 vs. 19,551 BAU/mL (difference 16,667 BAU/mL, p < 0.0001); IgG 2270 vs. 2932 BAU/mL (difference 660 BAU/mL, p = 0.04); and N-Ab 3.52 vs. 26.4 µg/mL (difference 21.4 µg/mL, p < 0.0001). Only IgM showed a lower peak post-booster antibody titer (COI 2.11 vs. 0.23, difference 1.63, 95% CI 1.05 to 2.38, p < 0.0001). By 180−210 days after the second or third vaccination, total S-Ab/IgG/N-Ab had decreased by 68.7/93.8/73.6% vs. 82.8/86.3/79.5%. The half-lives of IgG and N-Ab antibodies were longer after the third vaccination (IgG: 65 vs. 34 days, N-Ab: 99 vs. 78 days). Conclusion: Total S-Ab/IgG/N-Ab showed a greater increase post-booster, with IgG/N-Ab having a longer half-life.

Evaluation and Validation of the Roche Elecsys SARS-CoV-2 Antigen Electro-Chemiluminescent Immunoassay in a Southeast Asian Region.

INTRODUCTION: SARS-CoV-2 antigen tests can complement and substitute for RT-PCR tests. Centralized laboratory automated SARS-CoV-2 antigen tests that can be scaled to process a large number COVID-19 cases simultaneously are now available. We have evaluated the new Roche Elecsys SARS-CoV-2 antigen electro-chemiluminescent immunoassay. METHODS: The Roche SARS-CoV-2 antigen assay is a double-antibody sandwich electro-chemiluminescent immunoassay, which reports a cut-off index (COI) (COI ≥ 1.0 considered positive). We assessed assay precision and linearity, and confirmed the reactivity limit. We determined the assay sensitivity and specificity with a verification group (289 controls and 61 RT-PCR positive COVID-19 cases). Assay performance was also validated against the consecutive samples we received (7657 controls and 17 cases) for SARS-CoV-2 antigen testing from June to October 2021. RESULT: The assay had a within-run precision CV of 3.0% at COI 0.68, and a CV of 1.5% at COI 3.49. Between-run precision was 3.0% at COI 0.68 and 1.8% at COI 3.49. The assay was linear from COI 0.65 to 7.84. All 35 C50 ± 20% test results performed over 7 days were positive/negative, respectively. In the verification group, overall sensitivity was 42.6% (26/61 positive, 95% CI 30.0-55.9), and specificity was 99.7% (1/289 positive, 95% CI 98.1-100). The agreement between the SARS-CoV-2 antigen and the RT-PCR cycle threshold (Ct) count was good (r = 0.90). In cases with Ct counts ≤ 30, the antigen assay sensitivity improved to 94.7% (18/19 positive, 95% CI 74.0-99.9). In our validation group, antigen sensitivity was 62.5% (5/8 antigen positive, 95% CI 24.5-91.5) within the first week of disease onset, but no cases were reactive after the first week of disease onset. CONCLUSION: The Elecsys SARS-CoV-2 antigen assay has good performance within manufacturer specifications. The sensitivity of the Roche antigen assay was greatest when used in patients with lower RT-PCR Ct values (≤30) and within the first week of disease onset.

SARS-CoV-2 Spike and Neutralizing Antibody Kinetics 90 Days after Three Doses of BNT162b2 mRNA COVID-19 Vaccine in Singapore.

BACKGROUND: We evaluated the post-booster (BNT162b2) antibody responses in Singapore. METHODS: Participants (n = 43) were tested pre-booster and 20/30/60/90 days post-booster. Participants were boosted 120-240 days (mean 214 days) after their second dose and had no history or serologic evidence of prior COVID-19 infection; all participants had undetectable SARS-CoV-2 nucleocapsid antibodies throughout the study. Total nucleocapsid and spike antibodies (S-Ab) were assessed on the Roche Elecsys e802 and neutralizing antibody (N-Ab) on the Snibe quantitative N-Ab assay. RESULTS: Pre-booster median S-Ab/N-Ab titers were 829 BAU/mL/0.83 µg/mL; 2 participants were below manufacturer's N-Ab cut-offs of 0.3 µg/mL (0.192 and 0.229). Both S-Ab and N-Ab titers peaked at 30 days post-booster (median S-Ab 25,220 BAU/mL and N-Ab 30.3 µg/mL) at 30-37× pre-booster median levels. These peak post-booster S-Ab/N-Ab titers were 11× (25,220 vs. 2235 BAU/mL) and 9× (30.3 vs. 3.52 µg/mL) higher than the previously reported peak post-second dose levels. Antibody titers declined to 12,315 BAU/mL (51% decrease) and 14.3 µg/mL (53% decrease) 90 days post-booster. Non-linear regression estimates for S-Ab/N-Ab half-lives were 44/58 days. At 180 days post-booster, S-Ab/N-Ab are estimated to be 2671 BAU/mL/4.83 µg/mL. CONCLUSIONS: Both S-Ab and N-Ab show a good response following post-booster vaccination, with half-lives that may provide a prolonged antibody response.

Performance of the Abbott Architect Immuno-Chemiluminometric NT-proBNP Assay.

Background: We evaluated the performance of the Abbott N-terminal pro-brain natriuretic peptide (NT-proBNP) assay against the Roche NT-proBNP immunoassay across two sites. Methods: Precision, linearity, and sensitivity studies were performed. A combined method of comparison and regression analysis was performed between the Roche and Abbott assays using samples from both sites (n = 494). To verify biotin interference, lyophilised biotin powder was reconstituted and spiked into serum samples at two medical decision levels (final concentration 500/4250 ng/mL) and compared to controls. NT-proBNP was also measured in anonymised leftover sera (n = 388) in a cardio-renal healthy population and stratified into three age bands­<50 (n = 145), 50−75 (n = 183) and >75 (n = 60). Results: Between-run precision (CV%) for NT-proBNP was 4.17/4.50 (139.5/142.0 pg/mL), 3.83/2.17 (521.6/506.3), and 4.60/2.51 (5053/4973), respectively. The assay was linear from 0.7−41,501 pg/mL. The limit of blank/quantitation was 1.2/7.9 pg/mL. The assay showed no interference from biotin up to 4250 ng/mL. Passing−Bablok regression analysis showed excellent agreement between the two assays (r = 0.999, 95% CI 0.999 to 0.999, p < 0.0001). The Roche assay had a slightly persistent, negative bias across different levels of NT-proBNP. ESC age cut-offs for diagnosing acute heart failure are applicable for the Abbott assay, with the median NT-proBNP of subjects < 50 years old at 43.0 pg/mL (range 4.9−456 pg/mL), 50−75 years old at 95.1 pg/mL (range 10.5−1079 pg/mL), and >75 years old at 173.1 pg/mL (range 23.2−1948 pg/mL). Conclusions: The Abbott Architect NT-proBNP assay has good performance that agrees with the manufacturer's specifications. ESC/AHA recommended NT-proBNP age groups for acute heart failure diagnosis are applicable to this assay.

Neutralizing and Total/IgG Spike Antibody Responses Following Homologous CoronaVac vs. BNT162b2 Vaccination Up to 90 Days Post-Booster.

INTRODUCTION: We documented the total spike antibody (S-Ab), IgG S-Ab and neutralizing antibody (N-Ab) responses of BNT162b2/CoronaVac vaccinees up to 90 days post-booster dose. METHODS: We included 32 homologous regimen CoronaVac vaccinees and 136 BNT162b2 mRNA vaccinees. We tested their total S-Ab (Roche), IgG (Abbott) and N-Ab (Snibe) levels at set time points from January 2021 to April 2022. All subjects were deemed to be COVID-19-naïve either via clinical history (CoronaVac vaccinees) or nucleocapsid antibody testing (BNT162b2 vaccinees). RESULTS: All antibodies peaked 20-30 days post-inoculation. In BNT162b2 vaccinees, all post-booster antibodies were significantly higher than second-dose peaks. In CoronaVac vaccinees, IgG showed no significant differences between peak third-/second-dose titers (difference of 56.0 BAU/mL, 95% CI of -17.1 to 129, p = 0.0894). The post-vaccination titers of all antibodies in BNT162b2 vaccinees were significantly higher than those in CoronaVac vaccinees at all time points. Post-booster, all antibodies declined in 90 days; the final total/IgG/N-Ab titers were 7536 BAU/mL, 1276 BAU/mL and 12.5 µg/mL in BNT162b2 vaccinees and 646 BAU/mL, 62.4 BAU/mL and 0.44 µg/mL in CoronaVac vaccinees. CONCLUSION: The mRNA vaccine generated more robust total S-Ab, IgG and N-Ab responses after the second and third vaccinations.

Kinetics of the Neutralizing and Spike SARS-CoV-2 Antibodies following the Sinovac Inactivated Virus Vaccine Compared to the Pfizer mRNA Vaccine in Singapore.

INTRODUCTION: We compared the early total spike antibody (S-Ab) and neutralizing antibody (N-Ab) responses to two vaccines. METHODS: We studied 96 Pfizer and 34 Sinovac vaccinees over a 14-month period from January 2021 to February 2022. All vaccinees received three doses of one type of vaccine. Antibody levels (Roche Elecsys total S-Ab and the Snibe N-Ab) were tested 10 days after the first dose, 20 days after the second dose, and 20 days after the booster dose. RESULTS: At all time points, the mRNA vaccine generated higher S-Ab and N-Ab responses than the inactivated virus vaccine (S-Ab: first dose 2.48 vs. 0.4 BAU/mL, second dose 2174 vs. 98 BAU/mL, third dose 15,004 vs. 525 BAU/mL; N-Ab: first dose 0.05 vs. 0.02 µg/mL, second dose 3.48 vs. 0.38 µg/mL, third dose 19.8 vs. 0.89 µg/mL). mRNA vaccine recipients had a 6.2/22.2/28.6-fold higher S-Ab and 2.5/9.2/22.2-fold higher N-Ab response than inactivated virus vaccine recipients after the first/second/third inoculations, respectively. Mann-Whitney U analysis confirmed the significant difference in S-Ab and N-Ab titers between vaccination groups at each time point. CONCLUSIONS: The mRNA vaccines generated a more robust S-Ab and N-Ab response than the inactivated virus vaccine at all time points after the first, second, and third vaccinations.

Lipoprotein(a) as predictor of coronary artery disease and myocardial infarction in a multi-ethnic Asian population.

BACKGROUND AND AIMS: The role of Lp(a) in multi-ethnic Asian populations with coronary artery disease (CAD) has not been well established. The aims of this study were (i) to investigate whether Lp(a) is a predictor of CAD, and (ii) amongst patients with CAD, to ascertain whether Lp(a) is a predictor of acute myocardial infarction (AMI) and severity of CAD. METHODS: We compared three cardiovascular phenotypes from patients recruited at coronary angiography. CAD was defined as ≥50% coronary artery stenosis and subdivided into a group with AMI history (CAD+AMI+) and a group without (CAD+AMI-). Minimal CAD group (CAD-) was defined as normal or <30% coronary artery stenosis and no AMI. The severity of CAD was defined using the modified Gensini score. RESULTS: We studied 2025 patients comprising 94.5% men and 61.4% of Chinese ethnicity. The median Lp(a) level was highest in CAD+AMI+, followed by CAD+AMI- and CAD- (26.2, 20.1, and 15.8 nmol/L respectively). Similarly, the frequency of patients with Lp(a) ≥120 nmol/L were in the same order (11.8%, 9.1% and 2.4%). Lp(a) levels were highest among Asian Indians, followed by Malays and Chinese patients (p < 0.001). Lp(a) levels and Lp(a) ≥120 nmol/L were significant predictors of CAD (Odds ratio (OR) = 1.12 per 10 nmol/L increment, p < 0.001, and OR = 5.41 p = 0.004 respectively). Among patients with CAD, higher Lp(a) levels were associated with increased AMI risk (OR = 1.02 per 10 nmol/L increment, p = 0.024). Lp(a) ≥120 nmol/L was positively associated with CAD severity (p = 0.020). CONCLUSIONS: Plasma Lp(a) concentration is a positive predictor of CAD and AMI in a mostly male South East Asian population.

Markers of Focal and Diffuse Nonischemic Myocardial Fibrosis Are Associated With Adverse Cardiac Remodeling and Prognosis in Patients With Hypertension: The REMODEL Study.

BACKGROUND: The prognostic significance of focal and diffuse myocardial fibrosis in patients with cardiovascular risk factors is unclear. METHODS: REMODEL (Response of the Myocardium to Hypertrophic Conditions in the Adult Population) is an observational cohort of asymptomatic patients with essential hypertension. All participants underwent cardiovascular magnetic resonance to assess for myocardial fibrosis: nonischemic late gadolinium enhancement (LGE), native myocardial T1, postcontrast myocardial T1, extracellular volume fraction including/excluding LGE regions, interstitial volume (extracellular volume×myocardial volume), and interstitial/myocyte ratio. Primary outcome was a composite of first occurrence acute coronary syndrome, heart failure hospitalization, strokes, and cardiovascular mortality. Patients were recruited from February 2016 and followed until June 2021. RESULTS: Of the 786 patients with hypertension (58±11 years; 39% women; systolic blood pressure, 130±14 mm Hg), 145 (18%) had nonischemic LGE. Patients with nonischemic LGE were more likely to be men, have diabetes, be current smokers, and have higher blood pressure (P<0.05 for all). Compared with those without LGE, patients with nonischemic LGE had greater left ventricular mass (66±22 versus 49±9 g/m2; P<0.001), worse multidirectional strain (P<0.001 for all measures), and elevated circulating markers of myocardial wall stress and myocardial injury, adjusted for potential confounders. Twenty-four patients had primary outcome over 39 (30-50) months of follow-up. Of all the cardiovascular magnetic resonance markers of myocardial fibrosis assessed, only nonischemic LGE (hazard ratio, 6.69 [95% CI, 2.54-17.60]; P<0.001) and indexed interstitial volume (hazard ratio, 1.11 [95% CI, 1.04-1.19]; P=0.002) demonstrated independent association with primary outcome. CONCLUSIONS: In patients with hypertension, myocardial fibrosis on cardiovascular magnetic resonance is associated with adverse cardiac remodeling and outcomes.

Robust SARS-CoV-2 Antibody Responses in Asian COVID-Naïve Subjects 180 Days after Two Doses of BNT162b2 mRNA COVID-19 Vaccine.

BACKGROUND: Subjects with previous COVID-19 have augmented post-vaccination responses. However, the antibody response in COVID-naïve subjects from Southeast Asia is not well known. METHODS: 77 COVID-naïve vaccinees were tested with a full antibody panel [spike antibodies (total (T-Ab), IgG, IgM) and neutralizing antibodies (N-Ab)] pre-vaccination, 10 days after dose 1, and 20/40/60/90/120/150/180 days after dose 2. RESULTS: 10 days after dose 1, 67.6% (48/71)/69.0% (49/71) were T-Ab/IgG positive; only 15.5% (11/71)/14.1% (10/71) were N-Ab/IgM positive. While all (100%) subjects had brisk T-Ab, IgG and N-Ab antibody responses 20 days after complete vaccination, only 79.1% (53/67) were IgM positive. At 180 days (n = 8), T-Ab/IgG/N-Ab were still reactive (lowest T-Ab 186 U/mL, IgG 617 AU/mL, N-Ab 0.39 µg/mL), but IgM was negative in all samples. Spike antibody thresholds of T-Ab 74.1 U/mL (r = 0.95) and IgG 916 AU/mL (r = 0.95) corresponded to N-Ab reactivity (>0.3 µg/mL). Non-linear regression analysis showed that N-Ab would decrease to 0.3 µg/mL by 241 days, whereas T-Ab/IgG would need 470/163 days to reach titers of T-Ab/IgG associated with a N-Ab 0.3 µg/mL (76.4 U/mL and 916 AU/mL respectively). CONCLUSIONS: The antibody responses of T-Ab, IgG and N-Ab remain high and durable even at 180 days. N-Ab titers are expected to remain reactive up to 241 days post-vaccination.

Author Correction: High-Sensitivity cardiac Troponins in Cardio-Healthy Subjects: A Cardiovascular Magnetic Resonance Imaging Study.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

High-Sensitivitycardiac Troponinsin Cardio-Healthy Subjects: A Cardiovascular Magnetic Resonance Imaging Study.

The 99th percentile upper reference limits (URL) of high-sensitivity cardiac troponin (hs-cTn) in healthy subjects are essential for diagnosis and management of cardiovascular diseases. Unless screened stringently, subclinical disease affects the derived URL. In 779 healthy subjects(49% males; 17-88 years) screened by cardiovascular magnetic resonance (CMR), the gold standard for assessing cardiac volumes and myocardial mass; and estimated glomerular filtration rate (eGFR), the 99th percentile URL of hsTnT (Roche) and hs-cTnI (Abbott) were similar to the published URL. The overall 99th percentile URL of hsTnT and hsTnI were 15.2 and 21.2 ng/L, respectively; males had higher values than females (hsTnT: 16.8 versus 11.9 ng/L and hsTnI: 38.8 versus 14.4 ng/L). Correlation between hsTnT and hsTnI was modest (r = 0.45; p < 0.001). A larger proportion of healthy volunteers <60 years had detectable hsTnI compared to hsTnT (n = 534; 30.0% versus 18.3%, p < 0.001). Lower eGFR was an independent clinical determinant of hsTnT, but not hsTnI. Both hs-cTn concentrations were independently associated with myocardial mass and cardiac volumes (p < 0.01 for all), but only hsTnI was independently associated with CMR multi-directional strain measures and extent of LV trabeculations (p < 0.05 for all). Differences exist between hs-cTn assays and may influence their selection depending on cardiac conditions, patient population and local factors.

Measurement of cardiac troponin I in serum with a new high-sensitivity assay in a large multi-ethnic Asian cohort and the impact of gender.

BACKGROUND: As new high-sensitivity (hs) cardiac troponin (cTn) assays are introduced decision limits, based on the 99th percentile upper reference limit (99 percentile URL), for each method must be determined and from a sufficiently large cohort to mitigate against distortionary effects of high-end outliers. There is a paucity of studies with large multi-ethnic cohorts. METHODS: We determined the 99 percentile URL in 1120 (mean age: 50.4±8.2 y) apparently healthy (no history of diabetes, hypertension, heart, lung, or renal disease) Asians (597 men aged 35-65 y, 523 women aged 40-65 y) in a pre-market hs-cTnI assay (Abbott Diagnostics). RESULTS: Hs-cTnI performance was: limit of blank - 0.6 ng/l, limit of detection (LoD) - 1.5 ng/l; cTnI inter-assay coefficient of variation of 20% and 10% were 1.5 and 6.0 ng/l respectively. Hs-cTnI concentrations (range: 0-49.3 ng/l) were detectable (>assay LoD) in 92.3% of participants, and higher in men and individuals >50 y. All-subject, male, and female 99 percentile URL (90% CI) were 25.6 (19.6-32.6), 32.7 (21.1-47.9) and 17.9 (10.7-26.3) ng/l respectively. CONCLUSION: This hs-cTnI assay exhibits hs performance besides gender and age differences. The all-subject 99 percentile URL values are similar to those reported from some groups but not in others. Users need to establish their own decision limits.