A new assay might detect SARS-CoV-2 antigens with enough sensitivity and specificity to inform infection control measures and potentially inform novel, point-of-care testing methods, according to a recent Clinical Chemistry paper (Clin Chem 2021; doi:10.1093/clinchem/hvab158).

Many nucleic acid-based methods are sensitive enough to detect acute COVID-19 infections. However, persistent nucleic acid positivity after symptom resolution and disease recovery complicates infection control measures. This is true especially in immunocompromised patients who show long periods of nucleic acid positivity and have diverse presentations.

Previous research has shown that antigen results correlate better with viral culture results than nucleic acid determinations. These previous findings suggest that positive antigen results predict risk of transmission. However, most rapid antigen tests have low sensitivity and low positive percent agreement in high viral load cases, compared with real-time (rt) PCR.

The researchers developed the Microbubbling SARS-CoV-2 Antigen Assay (MSAA) as a screening test to identify patients who are likely to have active, ongoing replication and need close viral sequence monitoring. The assay relies on a smartphone’s camera and readout and has a limit of detection (LOD) of 0.5 pg/mL (10.6 fmol/L) for the nucleocapsid antigen or 4,000 copies/mL inactivated SARS-CoV-2 virus in nasopharyngeal (NP) swabs. The authors also developed a computer vision and machine learning-based automatic microbubble image classifier to accurately identify positives and negatives.

In a clinical validation study on 372 residual clinical NP swabs from intensive care unit COVID-19 patients and immunocompromised COVID-19 patients, the researchers compared MSAA and rtPCR performance.

Positive MSAA results agreed with positive rtPCR results, and negative MSAA results agreed with negative PCR results, at rates of 97%. In patients who were not immunocompromised, swabs’ antigen positivity rate decreased as days-after-symptom-onset increased, despite persistent nucleic acid positivity. MSAA detected antigens for longer, variable periods of time in immunocompromised patients with blood cancers. MSAA also detected viral sequence variations in patients with long duration of high antigen burden. Total microbubble volume—a quantitative marker of antigen burden—correlated inversely with cycle threshold values and days-after-symptom-onset.

The MSAA can provide insights into antigen dynamics in various patient populations, the researchers wrote.


Elevated maternal serum total bile acids are associated with increased risk of intrauterine growth restriction (IUGR) and low birth weight (LBW), which appears higher in pregnant individuals with hypertensive disorders, according to a recent study (JAMA Netw Open 2021; doi:10.1001/jamanetworkopen. 2021.17409).

Bile acids play essential roles in metabolic modulation. Excessive serum total bile acid (sTBA) levels during pregnancy are associated with adverse perinatal outcomes. However, their association with the risk of intrauterine growth restriction (IUGR) has been unclear.

The researchers conducted a retrospective cohort study that included regular prenatal exam records on 68,245 singleton pregnancies—delivered from 2014 to 2018—at a hospital-based center in Shanghai, China.

Nonlinear regression models suggested an inverted J-shaped association between maternal sTBA level during pregnancy and fetal birth weight, with a steep decrease in birth weight at high sTBA levels. The estimated mean birth weights of infants born to mothers with sTBA of 40.8 μg/mL, 0.4 μg/mL, and 4.1 μg/mL were 2,879 g, 3,290 g, and 3,334 g, respectively.

The researchers observed lower birth weight and a higher incidence of IUGR in patients with gestational elevated bile acids, known as gestational hypercholanemia (sTBA of 4.08 μg/mL or higher). The estimated mean birth weight of infants born to mothers with gestational hypercholanemia was 3,309 g, versus 3,338 g for infants born to mothers without the disorder. Incidence of IUGR among women with gestational hypercholanemia was 1.4%, compared with an incidence of 0.5% among women without gestational hypercholanemia.

Compared with patients with sTBA concentrations less than 4.08 mg/mL, those with gestational hypercholanemia had increased risk of LBW (with an adjusted odds ratio of 1.29), and IUGR (with an adjusted odds ratio of 2.18). The highest risk for LBW and IUGR was among pregnant individuals with both hypertensive disorders and hypercholanemia, compared with those with sTBA concentrations less than 4.08 μg/mL. For LBW, the adjusted odds ratio was 9.13. For IUGR, the adjusted odds ratio was 19.14. 

Clinicians should monitor bile acid concentration during follow-up for pregnancies with potential IUGR, the researchers wrote. They call for more research to confirm both their findings and benefits of drugs that lower sTBA concentrations.


A multitarget fecal immunochemical test (FIT) better detects advanced colorectal neoplasia and adenomas than an older FIT assay, a recent study found (Ann Int Med 2021; doi:10.7326/M20-8270). Many screening programs use the older FIT because of its overall performance characteristics, logistics, and cost-effectiveness, but its sensitivity for cancer precursors, including advanced adenomas (AAs) and advanced serrated polyps (ASPs), is far lower than its sensitivity for cancer.

The researchers developed a new, protein-based multitarget FIT (mtFIT) using a combination of hemoglobin, calprotectin, and serpin family F member-2. This was intended to outperform FIT in the detection of advanced neoplasia (AN) at an equally high specificity. The researchers tested the assay on leftover FIT material in 1,284 participants from a screening and referral population, classified by their most advanced lesion. Forty-seven had colorectal cancer, 135 had AA, 30 had ASP, 250 had nonadvanced adenoma, and 53 had nonadvanced serrated polyps. The study also included 769 controls.

The researchers used classification and regression tree (CART) analysis on biomarker concentration to identify the optimal combination for detecting AN. The researchers cross-validated the mFIT test’s performance using a leave-one-out approach and compared mFIT performance with a FIT test with equal specificity.

The CART analysis showed that mFIT had a cross-validated sensitivity for advanced neoplasia of 42.9%, versus 37.3% for FIT. The two tests both had sensitivity of 96.6%. In particular, cross-validated sensitivity for AAs increased from 28.1% to 37.8%. Using these results, the researchers performed an early health technology assessment that showed that screening via mFIT may be more cost-effective than FIT.

The researchers noted that their study is limited by enrichment with a referral population. A prospective screening trial of mFIT is being prepared, they added.