Dried blood spots have clear advantages over mini tubes when it comes to lab analysis of home collected samples for HIV and syphilis testing

By | December 25, 2020

Blood samples taken at home and sent by post to labs to screen for HIV and syphilis are more likely to be successfully processed when the collection technique used is dried blood spots rather than mini tubes, investigators from the UK report in Sexually Transmitted Infections. Close to 95% of dried blood spot samples were successfully checked in the lab compared to only 54% of samples returned using mini tubes. There was a lower rate of false positives using dried blood spots for both HIV and syphilis.

“Dried blood spot collection for HIV and syphilis has significantly better processing and analysis rates than mini tubes,” comment the authors. “False-positive results for HIV and syphilis in the mini-tube samples were high compared to dried blood spots.”

The research team, which was led by Dr Matthew Page of University Hospitals Birmingham, believe their results could have wider implications for home sampling for blood-borne infections, including viral hepatitis.

Glossary

syphilis

A sexually transmitted infection caused by the bacterium Treponema pallidum. Transmission can occur by direct contact with a syphilis sore during vaginal, anal, or oral sex. Sores may be found around the penis, vagina, or anus, or in the rectum, on the lips, or in the mouth, but syphilis is often asymptomatic. It can spread from an infected mother to her unborn baby.

false positive

When a person does not have a medical condition but is diagnosed as having it.

sample

Studies aim to give information that will be applicable to a large group of people (e.g. adults with diagnosed HIV in the UK). Because it is impractical to conduct a study with such a large group, only a sub-group (a sample) takes part in a study. This isn’t a problem as long as the characteristics of the sample are similar to those of the wider group (e.g. in terms of age, gender, CD4 count and years since diagnosis).

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statistical significance

Statistical tests are used to judge whether the results of a study could be due to chance and would not be confirmed if the study was repeated. If result is probably not due to chance, the results are ‘statistically significant’. 

reactive

Because of the possibility that a positive result from a single HIV test is, in fact, a false positive, the result is described as ‘reactive’ rather than ‘positive’. If the result is reactive, this indicates that the test has reacted to something in the blood and needs to be investigated with follow-up tests.

Home sampling for HIV and other sexually transmitted infections is now a well-established part of UK sexual health services, especially in regions with high HIV incidence and prevalence. Use has increased massively during the COVID-19 pandemic.

Testing kits are usually requested over the internet. Blood and urine samples are returned by post for processing in labs.

Blood-sample collection involves a person pricking a finger with a supplied sterile lancet. Blood is collected in a mini tube, or alternatively onto a filter paper, where the blood sample is absorbed and provides a dried blood spot.

About 500μl of blood is needed for labs to have a good chance of testing samples for both HIV and syphilis. But less than half this quantity is needed for the laboratory analysis of dried blood spots.

Dr Page and his colleagues had previously shown that dried blood spot testing for HIV had much better processing and analysis rates compared to mini tubes. But their research was restricted to HIV testing and only included 550 people.

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They therefore designed a much larger study that compared the performance of home sampling using mini tubes and dried blood spots for both HIV and syphilis testing.

The study population included 5670 individuals in the Midlands region of England who requested a home sampling kit for HIV and STIs between September 2016 and early January 2019. For the first year of the study, blood samples were obtained using mini tubes, then the method of collection switched to dried blood spots. In the laboratory, samples were tested for HIV using a fourth generation antigen/antibody test and for syphilis using an antibody test.

The researchers compared the performance of mini tubes and dried blood spots in terms of return rate, successful processing of results and proportion of false positives.

Overall, 1515 people home sampled using mini tubes and 4155 using dried blood spots.

There were no major differences in the characteristics of the two groups: 68% were female, 74% White British, 88% heterosexual and the average age was 26 years.

The return rate was 75% for kits that included dried blood spots compared to 71% for kits including mini tubes. When it came to processing samples in the lab, the results clearly favoured dried blood spots: 95% of returned dried blood spot samples were successfully processed, compared to 54% of mini-tube samples, a statistically significant difference (p < 0.001).

The main reason for unsuccessful processing was that blood samples weren’t of sufficient quantity for testing. This was much more of a problem with mini tubes compared to dried blood spots (32% vs 2%) and often led to technicians testing for HIV alone rather than both infections.

“The main reason for unsuccessful processing of mini tube samples was that blood samples weren’t of sufficient quantity for testing.”

In some instances, the blood obtained had degraded (haemolysis) making accurate testing impossible, and this problem was much more likely to occur with mini tubes (7%) compared to dried blood samples (0%).

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Of the successfully analysed blood, 6% of mini-tube samples and 1% of dried blood spots yielded a result that was reactive for HIV. Confirmatory tests in a clinic using samples of blood drawn from a vein showed false-positive rates of 5% from mini tubes and 0.4% for dried blood spots – this difference was statistically significant (p < 0.001).

As regards syphilis, 1% of mini-tube samples and 0.7% of dried blood spots were reactive. Follow-up testing in the clinic showed that 0.4% of mini-tube samples and 0% of dried blood spots were false positives.

The researchers believe their results show clear advantages of dried blood spots in terms of processing and accuracy, although laboratory testing of dried blood spots is slightly more complicated and costly.

“A postal sampling service using dried blood spots to screen for HIV, syphilis and potentially other blood-borne viruses could be a viable option in a post-COVID era,” conclude Dr Page and his co-authors.

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