Introduction: Who Should Consider Diagnostic Testing for PNH

If you notice unusual symptoms like dark-colored urine in the morning, persistent fatigue, or unexplained shortness of breath, it may be time to seek medical evaluation. Paroxysmal nocturnal hemoglobinuria can be difficult to recognize because its symptoms often overlap with other conditions. Many people experience signs gradually, which can delay diagnosis significantly.^[1]

Anyone experiencing unusual blood-related symptoms should consider speaking with their healthcare provider about testing. This is particularly important for people who already have bone marrow disorders such as aplastic anemia (a condition where the bone marrow doesn’t make enough blood cells) or myelodysplastic syndrome (a group of disorders where the bone marrow produces abnormal blood cells). These individuals have a higher likelihood of developing PNH and should be monitored more closely.^[1]

The timing of diagnosis matters greatly. Unfortunately, fewer than 40 percent of people with PNH receive a diagnosis within the first year after symptoms begin, and nearly one-quarter of all diagnoses take five years or longer. This delay can happen because symptoms appear gradually and are common to many other diseases, making PNH easy to overlook.^[5]

Early detection through appropriate testing can make a significant difference in outcomes. When PNH is identified promptly, treatment can begin sooner, potentially preventing serious complications such as blood clots, which historically have been the main cause of death in people with this condition.^[14]

Classic Diagnostic Methods for Identifying PNH

Diagnosing PNH begins with recognizing the pattern of symptoms and understanding your medical history. Your doctor will want to know about any episodes of dark urine, how often you feel tired or weak, whether you’ve had blood clots, and if you have any existing bone marrow conditions. A physical examination can reveal signs like pale skin from anemia or small red dots on the skin that indicate bleeding beneath the surface.^[1]

The foundation of PNH diagnosis relies on blood tests. Standard blood work can reveal important clues. A complete blood count may show low numbers of red blood cells (indicating anemia), low white blood cells (suggesting infection vulnerability), or low platelets (which can cause bleeding problems). Additional blood tests might detect elevated levels of a substance called lactate dehydrogenase or LDH, which rises when red blood cells break down. The presence of free hemoglobin in the blood—hemoglobin that has been released from destroyed red blood cells—is another key indicator.^[1]

Testing your urine can provide valuable information as well. When hemoglobin from broken-down red blood cells passes through your kidneys and into your urine, laboratory analysis can detect it. This is what causes the characteristic dark color that gives the disease part of its name. However, it’s important to understand that not all people with PNH will have visible dark urine, especially in the early stages or in cases where the disease activity is lower.^[1]

Flow Cytometry: The Gold Standard Test

The most accurate and reliable way to diagnose PNH is through a specialized laboratory technique called flow cytometry. This test examines your blood cells to determine whether they are missing the protective proteins that normally shield them from attack by your immune system’s complement system (a part of the immune system that helps fight infections but can mistakenly attack the body’s own cells in PNH).^[2]

In PNH, a genetic change in bone marrow stem cells prevents the production of a protein anchor called GPI (glycosylphosphatidylinositol). This anchor normally holds protective proteins called CD55 and CD59 on the surface of blood cells. Without these proteins, red blood cells become vulnerable to destruction by the complement system. Flow cytometry can identify which blood cells are missing these protective markers and measure what percentage of your blood cells are affected.^[2]

The flow cytometry test works by using special fluorescent markers that attach to normal surface proteins. When your blood sample is analyzed under the flow cytometer, cells with normal proteins will light up, while PNH cells lacking these proteins will not. This allows laboratory specialists to count how many of your blood cells are affected and to what degree, which helps doctors understand the severity of your condition.^[13]

Historical Tests and Their Limitations

Before flow cytometry became widely available, doctors used other tests to diagnose PNH. One of the oldest is the Ham test, also called the acidified serum test, which was developed in 1937. In this test, a patient’s red blood cells are mixed with acidified serum in a test tube. If the cells break apart in this acidic environment, it suggests PNH. While this test was groundbreaking in its time, it is less accurate and less sensitive than modern flow cytometry methods.^[2]

Another historical test is the sucrose hemolysis test, which works on a similar principle by exposing red blood cells to a low-salt solution to see if they break down more easily than normal cells. Both of these older tests have largely been replaced by flow cytometry in modern medical practice because the newer method provides much more detailed and reliable information about the disease.^[13]

Distinguishing PNH from Other Conditions

Because PNH symptoms can resemble those of many other blood disorders, your doctor will work to rule out alternative diagnoses. The combination of hemolysis (red blood cell destruction), low blood cell counts, and a tendency to form blood clots is particularly suggestive of PNH. However, these features can also appear in other conditions, which is why specific testing is essential.^[4]

Your doctor may perform additional tests to check for other causes of anemia or blood cell problems. This might include testing for vitamin deficiencies, checking kidney and liver function, looking for signs of internal bleeding, or evaluating for other immune system disorders. Bone marrow biopsy, where a small sample of bone marrow is removed and examined under a microscope, may be performed to assess whether you have underlying bone marrow failure or another bone marrow condition alongside PNH.^[6]

Imaging studies such as ultrasound or CT scans (computed tomography scans that create detailed cross-sectional images of the body) might be ordered if your doctor suspects blood clots, particularly in unusual locations like the liver or abdominal blood vessels, which are more common in PNH than in other conditions.^[4]

Diagnostic Testing for Clinical Trial Qualification

When patients with PNH are being considered for participation in clinical trials, they must undergo a standardized set of diagnostic tests to determine eligibility. These requirements ensure that researchers are studying similar groups of patients and that results can be compared reliably across different studies.

The central requirement for clinical trial enrollment is confirmation of PNH diagnosis through flow cytometry. Trial protocols typically specify a minimum percentage of PNH cells that must be present in the blood. For example, some studies might require that at least 10 percent of a patient’s red blood cells or white blood cells show the characteristic absence of GPI-anchored proteins. This threshold ensures that enrolled patients have clinically significant disease rather than very small populations of abnormal cells that might not cause symptoms.^[13]

Clinical trials also require documentation of disease activity and its impact on the patient. This often involves measuring markers of hemolysis in the blood. Elevated LDH levels, reduced haptoglobin (a protein that normally clears free hemoglobin from the blood), or the presence of free hemoglobin in blood samples can all indicate active red blood cell destruction. These measurements help researchers identify patients who might benefit most from the treatment being studied.^[7]

Complete blood counts are standard requirements for trial enrollment. Researchers need to know your baseline numbers of red blood cells, white blood cells, and platelets before treatment begins so they can measure how well the experimental therapy works. A history of blood transfusions is also carefully documented, as transfusion dependence (needing regular blood transfusions to maintain adequate red blood cell levels) is an important indicator of disease severity.^[8]

Many clinical trials for PNH also require screening for a history of blood clots. Because thrombosis is one of the most serious complications of PNH, researchers want to understand whether new treatments can prevent this life-threatening problem. Patients may undergo imaging studies to check for existing clots, and their medical records are reviewed for any previous thrombotic events.^[8]

Kidney function tests are commonly required in PNH clinical trials because the disease can affect the kidneys over time. The breakdown products from destroyed red blood cells can damage kidney tissue, leading to reduced kidney function or even chronic kidney disease (long-term kidney damage that can progress over time). Tests measuring creatinine levels and other markers of kidney health help researchers understand each patient’s baseline kidney function and monitor for any changes during the trial.^[1]

Some trials require testing for specific genetic variations that might affect how patients respond to treatment. For instance, certain variations in complement system genes can influence disease severity or treatment response. While these genetic tests are typically performed by the research team and may not be part of routine clinical care, understanding that you might need them can help you prepare for the enrollment process.^[10]

Before starting some experimental treatments, particularly those that affect the immune system, patients must be up to date on certain vaccinations. This is especially important for treatments that block parts of the complement system, as patients become more vulnerable to specific bacterial infections, particularly meningococcal disease (a serious bacterial infection that can cause meningitis or bloodstream infections). Documentation of vaccination status is a standard part of clinical trial screening.^[10]