Medical imaging & patient care

Tick in time

The majority of laboratory tests performed for the diagnosis of Lyme disease are based on detection of the antibody responses against Borrelia Burgdorferi in serum. But as the sensitivity of such tests increase with the duration of the infection, patients early in their illness are more likely to register a false negative result. There is a need to simplify the testing algorithm for the condition, improving sensitivity in early disease while maintaining high specificity. We speak to Dr Adriana Marques, National Institutes of Health, about how this might be accomplished.

Throughout the US, rates of Lyme disease are climbing. With over 300,000 new cases recorded every year – around triple the rate from two decades ago – the disease is now the most commonly reported vector borne illness in the United States.

As it becomes more rampant, it is also becoming more geographically widespread. While incidence is still concentrated heavily in the northeast and upper Midwest, with 95% of confirmed cases reported from just 14 states, the ticks that carry the disease are growing in number and territory. One often-cited reason is climate change: global warming is thought to increase both the range in which the ticks can thrive, and the amount of time they have to feed.

For those at risk of infection, the need to take precautions is clear. Lyme disease, which often starts out with a ‘bullseye’ rash (erythema migrans) at the site of the tick bite, typically gives rise to an unpleasant array of symptoms ranging from fever and fatigue to muscle spasms, neurological problems and heart palpitations.

Following a course of antibiotics, most patients make a full recovery, especially if the infection is caught at its early stages. Unfortunately, reaching an early diagnosis is not always straightforward. As Dr Adriana Marques of the National Institutes of Health (NIH) explains, current laboratory methods are far from infallible.

“We have made major advances since the early days when Lyme disease was first recognised, but there are many areas in which we need further improvement,” she says. “As the rate of the disease increases – and the rate of differential diagnoses increases – there is a tremendous need to improve diagnostics.”

Early diagnosis

Marques, who heads up the clinical studies unit at the NIH’s Laboratory of Clinical Infectious Diseases, has conducted research into many aspects of the condition. A key point of focus is improving assays right from the onset of infection.

“One of the questions we’re exploring is improving early diagnosis of Lyme disease and developing a point of care test which could be run in the emergency room,” she says. “At the moment, you have to draw blood and send it out to a laboratory, but in certain situations, say when you have someone who presents with facial palsy, a point of care test would be very helpful.”

The problem with clinical testing at the early stages is that the vast majority of lab tests are based on antibody response. Should a person become infected with Borrelia Burgorferi, the bacterium that causes Lyme disease, it will take a few weeks before they develop enough antibodies to register on a test. This means that for newly infected patients, the test is likely to come out negative and any diagnoses will be need to be based on the physician’s observations.

“For patients with a possible erythema migrans rash and a compatible history of being exposed to ticks, that patient should be treated clinically; their symptoms do not require anything further to be offered therapy,” Marques says.

Once a few weeks have elapsed, the patient’s immune response starts to fire up and the test becomes vastly more sensitive. Unfortunately, it does not necessarily become more specific. In order to ascertain whether they really are suffering from Lyme disease, or one of several differential diagnoses, the clinician will need to factor in information such as the typicality of symptoms and geographic location.

“The predictive value of any given test will depend on its sensitivity and specificity, as well as the likelihood that the patient has Lyme disease,” says Marques. “So if you have a patient who has a very low chance of having Lyme disease, and you’re doing the test mostly to rule out the disease, a positive result is more likely to be a false positive than a true positive. Each subsequent positive test will increase its predictive value, and it will become much more likely that the test is a true positive.”

Holy grail

Given these acknowledged limitations, the holy grail would surely be a direct test, which checks for the presence of B. Burgdorferi itself and could theoretically enable diagnosis right from the point of infection.

This, however, has yet to become a reality. Marques points out that, while two direct testing methods have been developed – culture tests and polymerase chain reaction (PCR) – both come with significant drawbacks that curtail their diagnostic applications.

“The issue with direct tests for B. Burgdorferi is there’s very little of the bacteria in the body at the most stages of infection,” she says. “Culture is a very difficult technique which very few laboratories can do – it requires a special medium and it takes many weeks for the bacteria to grow. So culture is a test that’s important for research, but is not useful in the clinic.’

“PCR is helpful for evaluating the synovial fluid in patients with Lyme arthritis, but the chances of finding B. Burgdorferi either by culture or PCR are very low, so at this point they are not really useful in clinics. Maybe with new techniques there will be something you can use to look for the bacteria itself, but right now it’s very hard to find it.”

For the time being, US clinics abide by guidelines set by the Centres for Disease Control and Prevention (CDC). Since 1995, the CDC has recommended using a two-step testing process: an enzyme immunoassay to begin with, followed by the so-called ‘Western blot’ test in the case of a positive or indeterminate result. While there can be difficulties associated with interpretation, the process works relatively well when recommendations are followed to a T.

More concerning are the specialty laboratories – often marketing themselves as providing a second opinion – which use non-validated tests. Often, these miss out the first step and skip straight to the ‘Western blot’. Taken by itself, this approach can be highly unreliable. According to a recent study that examined four such labs, three of them generated false positives, and one had a false positive rate of higher than 50%.

“Patients with chronic subjective symptoms without a diagnosis can be vulnerable and desperate for an answer as to the cause of their illness. Giving them a false diagnosis based on flawed testing is misleading,” wrote Dr Dan Gregson, from the University of Calgary, in a commentary published in the Canadian Medical Association Journal in August.

As Marques explains, it would be a mistake to overemphasis the failings of current testing protocol as it is followed by public health providers, and therefore opt for private testing.

“If you add all the numbers together you get the impression that the tests are not very sensitive,” she says. “Yes, they are not very sensitive for people early in their illness who present with erythema migrans, but they work very well on patients who have had the illness for a while.”

In an ideal world, she says, there might be different tests offered for different stages of infection, adapting the testing algorithm to provide high sensitivity and specificity no matter how recently the bacterium was contracted.

“If a patient has a rash that could be erythema migrans you’d use one type of test, whereas a patient who presented later, the test might be different,” she suggests.

Recently, she has been funnelling her energies towards what is perhaps the trickiest diagnostic stage of all: persistent Lyme disease infection. Long after most Lyme disease sufferers are cured, a small but significant proportion continue to grapple with lingering symptoms. This condition – officially recognised as Post-Treatment Lyme Disease Syndrome (PTLDS) and more colloquially as ‘chronic Lyme disease’ – has been subject to much debate, with experts divided on what it is and how best to treat it.

Marques and her team have been aiming to find a biomarker for the condition, which might differentiate patients suffering PTLDS from those who are having complaints because of something else.

“The question is, can we find the bacteria itself after antibiotic therapy, and can we correlate that with having symptoms,” he says. “At this point we don’t have anything that could be used in clinics, but there might be something that will be able to be developed from the large screenings of biomarkers that are being done.”

As Lyme disease continues to grow in prevalence, it seems that current diagnostic techniques are due an overhaul. As Marques sees it, this might entail simplifying the algorithm into a single test or procedure, improving sensitivity for early disease detection, and creating tests that work independently of disease duration, ideally at point of care. When such a test will reach the clinic, only time will tell, but for researchers breaking ground in this field, the endgame has been defined.

 

This article appears in the Vol 2 2015 edition of Practical Patient Care

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s