Dried blood spots are a valid alternative to venipuncture for COVID-19 antibody testing

Abstract

Background: Serologic analysis is an important tool towards assessing the humoral response to COVID-19 infection and vaccination. Numerous serologic tests and platforms are currently available to support this line of testing. Two broad antibody testing categories are point-of-care lateral flow immunoassays and semi-quantitative immunoassays performed in clinical laboratories, which typically require blood collected from a finger-stick and a standard venipuncture blood draw, respectively. This study evaluated the use of dried blood spot (DBS) collections as a sample source for COVID-19 antibody testing using an automated clinical laboratory test system.

Methods: Two hundred and ninety-four participants in the BLAST COVID-19 seroprevalence study (NCT04349202) were recruited at the time of a scheduled blood draw to have an additional sample taken via finger stick as a DBS collection. Using the EUROIMMUN assay to assess SARS-CoV-2 anti-spike IgG status, DBS specimens were tested on 7, 14, 21, and 28 days post- collection and compared to the reference serum sample obtained from a blood draw for the BLAST COVID-19 study.

Results: SARS-CoV-2 anti-spike IgG status from DBS collections demonstrated high concordance with serum across all time points (7-28 days). However, the semi-quantitative value from DBS collections was lower on average than that from serum, resulting in increased uncertainty around the equivocal-to-positive analytical decision point.

Conclusions: DBS collections can be substituted for venipuncture when assaying for COVID-19 IgG antibody, with samples being stable for at least 28 days at room temperature. Finger-stick sampling can therefore be advantageous for testing large populations for SARS-CoV-2 antibodies without the need for phlebotomists or immediate processing of samples. We have high confidence in serostaus determination from DBS collections, although the reduced semi-quantitative value may cause some low-level positives to fall into the equivocal or even negative range.

Keywords: COVID-19; COVID-19 antibody; Dried blood spot; SARS-CoV-2 IgG.

Fig. 1

Agreement between dried blood spot (DBS) and reference serum at 7 days post-collection A. Agreement between qualitative results, where each OD ratio was categorized as being IgG negative, IgG positive or equivocal based on the manufacturer's test cutoff values. Only the first DBS spot analyzed 7 days post-collection is presented, and the agreement is summarized using Cohen's κ. B. Association between quantitative results, showing the Pearson correlation between DBS and reference serum OD ratio for the first DBS spot analyzed 7 days post-collection. C. Differences between DBS and reference serum OD ratios versus the average ratios using all spots per study participant 7 days post-collection. PA = phlebotomist acquired sample; Self = self-stick acquired sample.

Fig. 2

Differences between repeat and reference serum OD ratios at 7 days post-collection for dried blood spots (DBS) and serum measurements. A. The difference between the mean OD ratio from a DBS collection (three measurements) and the OD ratio of the reference serum from the same subject (y-axis values) as a function of the reference serum OD ratio (x-axis). The results shown are based on a linear mixed model analyzing the three repeated OD ratios for each sample from both the DBS and Serum Repeat (SR) studies. The means (dark lines) were estimated using a linear mixed model, and 95% confidence intervals are shown as the shaded area. The red line at the bottom indicates the region where the difference from the reference serum OD ratio differs significantly between the DBS and SR studies. B. Estimated mean differences as indicated in panel A, but at reference serum measurements near the analytical decision points (OD ratios of <0.8, 0.80–1.09 and ≥ 1.1 are interpreted as negative, equivocal, and positive, respectively). Estimated means and 95% confidence intervals are shown. The red line at the bottom indicates a statistically significant difference between serum and DBS. C. The estimated variance among replicate OD ratio values for all subjects, with the variance depending on the reference serum OD ratio. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Comparison of DBS vs. serum collection on serostatus determination for specimens tested 7 days after collection. A. Serum OD ratios highlighting discordant qualitative results when measured using DBS. B. DBS OD ratios highlighting discordant qualitative results when measured using serum. C. DBS OD ratio for OD ratios ≤ 1.3. Orange and blue dots represent discordant qualitative results between serum and DBS. All other data points are concordant between the two cohorts. Red lines indicate the assay's analytical decision points (0.8, 1.1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Impact of time between sample collection and processing on assay OD ratios for phlebotomist acquired DBS collections. A. Mean difference between DBS and reference serum collections from testing performed in triplicate as a function of reference serum OD ratio. Each time point (i.e. 7, 14, 21 and 28 days post-collection) is denoted by a different colored line. B. Mean difference between DBS and reference serum collections from testing performed in triplicate at OD ratios of 0.8 and 1.1 (analytical decision cutoff values).