Blood Tests for Atrial Fibrillation

Atrial Fibrillation is diagnosed and monitored using pulse checks and ECGs. However, a series of blood tests 💉are also necessary to help individuals understand and manage their condition effectively. This post summarises the standard blood tests that should be undertaken when AF is diagnosed and those required for ongoing surveillance. These are tailored to a UK perspective, but because human physiology is the same across the World, the rationale applies globally. Having said that, no individual is 'standard', so each batch of tests and their frequency should be personalised based on age, pre-existing conditions and the results of previous blood tests. Your doctor should oversee this process but the below can form a starting point.

Initial Diagnosis

When AFib is diagnosed, blood tests should be performed for two purposes:

to understand the underlying health status to guide treatment
to identify any reversible factors that may have contributed to AF onset

Full Blood Cell (FBC) Count

Known as a Complete Blood Count (CBC) in the Americas, it measures the concentrations of various blood components. If you keep your eyes open during the blood draw, this sample goes into the purple bottle. Particularly relevant to Atrial Fibrillation, the FBC can show:

Anaemia: A low red blood cell count (haemoglobin < 13.5 g/dL in men or < 12 g/dL in women) is especially common in women, but it decreases in both men and women as they age. A study of nine million adults (2% of whom developed AF) registered in the Korean Health Insurance System found the risk of AF was higher in people with anaemia. This may be driven by reduced oxygen-carrying capacity of the blood to the heart or increased strain on the heart to pump enough oxygen around (the exact mechanism is unclear). Anaemia can also exacerbate the symptoms associated with AF, namely tiredness or shortness of breath, as anaemia can reduce and exacerbate this effect of AF. Detecting anaemia is important as it can often be reversed with supplementation and treatment of the underlying cause. Interestingly, the risk of AF also increases when Haemoglobin levels are above normal levels too.

This Kaplan-Meier risk curve is taken from the large Korean study. After controlling for other risk factors that may confound it, a clear association was seen between the risk of AF and anaemia. Image taken from here

Infection: Elevated white blood cell counts can also be detected on the FBC and indicate infection. In the in-hospital setting, new AF can complicate up to 5% of sepsis cases (shown in a study from Boston here). The AF can sometimes settle with the treatment of the underlying AF. In the outpatient setting, infections can also strain the body's circulation, cause inflammation, and derange circulating electrolytes, which decreases the threshold for AF.

Urea and Electrolytes

This is another routine blood test to check the levels of various minerals and waste products in the blood. It is also called a 'Renal Profile' or BUN (Blood Urea Nitrogen). 'This includes:

Potassium level: This occurs when levels fall below 3.5mmol/L. Low potassium levels increase the excitability of cells in the heart, making them more vulnerable to AF. This would be unusual to see on a routine blood test without a clear trigger but can result from significant bodily fluid shifts, such as dehydration after diarrhoea. If low potassium is detected after a new AF diagnosis, it is usually considered a contributing factor rather than a primary cause.
Creatinine: (Not to be confused with the powdered supplement Creatine!) This blood test is a measure of kidney function. Analysis of the same Korean Insurance Database showed that patients with Chronic Kidney Disease were at higher risk of AF than patients with normal kidney function. However, kidney failure is harder to treat than anaemia and can be irreversible. Poor kidney function also limits the medications that can be used in Atrial Fibrillation, so it is important to detect it before starting any drugs.

Thyroid Function Tests

Thyroid hormones can significantly influence heart rhythm and should be checked in every person with a new diagnosis of AF. An abnormal thyroid function is reversible and treatment can be enough to treat the AF.

Thyroid Function Tests: Traditionally, high blood thyroid hormone levels are associated with AF. It increases circulating adrenaline levels that can trigger AF while also lowering the threshold for AF events by making heart cells more excitable. Hyperthyroidism sets up a perfect storm for AF by also increasing the demand on the heart and causing structural changes in the heart chambers that sustain AF, too. There are further sub-tests needed to determine the cause for the thyroid dysfunction which guides the treatment strategy.

The other contenders

These blood tests have indirect value in the management of AF. In the UK, they aren't considered mandatory in the AF work-up toolkit, but I will make arguments for settings where they could add value:

B-type Natriuretic Peptide (BNP) Level: BNP or NTpro-BNP is a screening blood test for Heart Failure. It is, therefore, not always performed if an echo scan can be done instead, as this imaging test is the gold standard detection tool and is chosen in preference if available. However, it can sometimes be done if the waiting times for echo scans are long or in the context of research studies- like the AFHF study!

HbA1c: The HbA1c is the diagnostic test for Diabetes. The relationship between metabolic disease and AF is becoming increasingly clear. Undiagnosed Diabetes is found in X% of people on routine screening blood tests. Detection and subsequent treatment can help to restore blood sugar control and improve AF management too.

Cholesterol profile: Again, not directly aligned, but this is a good opportunity to get an oversight of cardiovascular risk factors.

Patients with AF often require long-term medications that necessitate regular blood monitoring to ensure efficacy and safety. Again, this list isn't exhaustive but can be used to prompt some considerations if you are taking them.

Warfarin

Warfarin reduces the stroke risk by thinning the blood. The INR (International Normalized Ratio) measures how long blood takes to clot and needs to be checked regularly to determine the correct dose. The target INR range for most AF patients is 2.0-3.0. Unfortunately, it is a delicate balance that can be disturbed by illness, other medications and even too much grapefruit juice! The Platelet Count is also an important marker of bleeding complications and should be checked at least once. Novel anticoagulants like Edoxaban, Rivaroxaban, Dabigatran, and Apixaban do not require such monitoring; however, the initial assessment of kidney function is important. Kidney function affects the clearance of these medications and can impact their efficacy and safety.

Digoxin

Digoxin is used for rate control in persistent AF. Digoxin levels are monitored to guide dosing. It can also be measured to check for toxicity. This should especially be done in the context of symptoms such as include nausea, dizziness, palpitations or visual disturbance.

Amiodarone

Amiodarone is effective for rhythm control in AF but has potential side effects that require regular monitoring. These include Thyroid and Liver Function, which should be checked at regular intervals.

Flecainide

Flecainide is excreted via the kidneys. So, ensuring normal kidney Function is To ensure proper drug excretion and prevent accumulation that could lead to toxicity.

Sotalol

In addition to the routine Urea and Electrolytes, Magnesium Levels should also be checked. Low magnesium levels can predispose patients to arrhythmias, and monitoring and correcting magnesium levels can help prevent AF recurrences.

Bisoprolol and Diltiazem/Verapamil

These medications, commonly used for rate control, generally do not require regular blood tests.

Conclusion

Blood tests are one component of effective monitoring of Atrial Fibrillation and the medications used to treat it. In practice, blood tests are done by all health care providers: Electrophysiologists, Hospitalists (if you are admitted), and primary care physicians, so results can be scattered. It is important for patients to have an understanding of what tests they should be expecting to have, and keeping a record of results can be empowering.