Capillary Blood Outperforms Venous Samples in Malaria Diagnosis

New research suggests that capillary blood samples are preferable to venous blood samples when attempting to detect malaria.

The study could change the way blood samples are taken in malaria-prone countries and also alter how researchers interpret data around malaria rates.

Researchers from Germany, Austria, and Gabon collaborated on the study, which involved quantifying parasitemia in capillary (CAP) and venous (VEN) samples, as well as evaluating the performance of the laboratory workers who were analyzing the samples. A quantitative polymerase chain reaction (qPCR) gold standard was used as the control against which to assess parasite counts and technician performance.

The results showed that capillary samples had 16.6% higher rates of parasitemia compared with venous samples. The data further showed an 8% sensitivity gap between the CAP and VEN samples, a gap that widened when dealing with low-level parasitemias.

Johannes Mischlinger, MD, MSc, DTM&H, of the Bernhard Nocht Institute for Tropical Medicine, in Hamburg, said the data show it would be preferable to simply using CAP samples for all malaria testing.

“From a scientific medical standpoint, I think we provided robust data in our study to support the universal usage of CAP blood for malaria diagnosis, as it is more sensitive to detect malaria,” he told Contagion^®. “As CAP blood sampling constitutes a cost-efficient optimization of malaria diagnostics it is difficult to anticipate any barriers at all.”

He said the cost-efficiency of CAP blood samples means it would be economically feasible, even in low-income countries, which are often the most burdened by malaria.

The findings don’t merely have an impact on individual diagnoses. Dr. Mischlinger noted that they could also have a major impact on how we track and attempt to eradicate malaria, in part because we don’t have any good data about which types of samples are being taken and recorded.

“Currently, no distinction is being made with regards to the blood source of the sample from which a given malaria episode was diagnosed,” he said. “It can be anticipated that the blood sources are almost exclusively CAP and VEN blood.”

However, whether CAP or VEN blood was taken in a given case is currently unknowable. That’s potentially a major problem, Dr. Mischlinger noted, since the degree of accuracy of testing could vary significantly depending on the type of blood used. He noted that around 200 million cases of malaria are reported to the World Health Organization each year. Hypothetically speaking, if half of those cases—100 million—were determined based on venous samples, the 8% sensitivity gap between CAP and VEN samples could mean millions of misdiagnoses.

“This could suggest that 8 million malaria cases were annually failed to be correctly diagnosed (due to the 8% CAP-VEN sensitivity gap),” he said. “Given the enormous global case burden of malaria even this small, but significant percentage may constitute thousands (to potentially millions) of missed cases each year, who continue to be infectious to mosquitoes.”

One other major problem noted by the study was the lack of technical proficiency among laboratory workers conducting the tests. The paper noted that for poorly trained workers, even small differences in parasitemia in CAP versus VEN blood could be the difference between catching and not catching malaria in a VEN sample.

“I think that the topic of under-skilled microscopists constitutes a big problem,” he said. “I do not know of any agencies that are working to improve microscopist training, I’m afraid.”

Dr. Mischlinger said more collaboration between researchers in low-income countries and resource-rich countries, WHO-based training campaigns, or private philanthropy are potential solutions to the problem of misdiagnoses.