Introduction

Paroxysmal nocturnal haemoglobinuria (PNH) is a rare, long-term blood disease that people are not born with but develop later in life. In this condition, the body’s own defence system (called the complement system), which normally protects us from infections, mistakenly attacks and destroys red blood cells.¹

PNH begins because of a random genetic change (mutation) in some stem cells in the bone marrow (which makes all blood cells). These altered cells multiply and gradually take over, producing red blood cells that are missing certain special protective proteins (called GPI-anchored complement regulatory proteins, such as CD55 and CD59) that normally sit on the cell surface and shield them from attack. These protective proteins are held in place by tiny hooks called GPI anchors. In PNH, the anchors are faulty, so the proteins fall off. Without their shields, red blood cells are left exposed, and the immune system destroys them.¹

This ongoing destruction of red blood cells(RBC), known as haemolysis, is the main problem in PNH. Another serious risk is the tendency to form blood clots (thrombosis).¹

Catching PNH early is very important because it helps guide treatment and improve long-term outcomes. The challenge is that symptoms vary a lot, so diagnosis is often delayed.²

Importance of lab tests in PNH

Lab tests are essential in PNH because they:

• Confirm the diagnosis
• Show treatment efficacy
• Help predict future risks¹

Doctors usually consider PNH if someone has:

• Ongoing red blood cell destruction not explained by other common causes
• Blood clots in unusual places (like the liver, brain, abdomen, or skin), especially when combined with haemolysis
• Unexplained low blood counts or bone marrow problems (such as aplastic anaemia or myelodysplastic syndrome)
• Repeated attacks of stomach pain or difficulty swallowing, especially with signs of haemolysis³

Other common issues in PNH include: 

• Extreme tiredness
• Erectile problems in men
• High blood pressure in the lungs (pulmonary hypertension)
• Kidney damage⁴

Routine tests for PNH

Doctors usually start with basic blood and urine tests available in almost all hospitals and clinics. No single test gives the full picture, so results are combined with symptoms to understand how much haemolysis is happening.¹

Haemoglobin (Hb)

It is the red blood cell protein that carries oxygen. It shows how anaemic a person is. Haemoglobin drops because RBCs are destroyed faster than they can be replaced, or the bone marrow is not making enough new ones. 

These values represent the severity of anaemia :

• Mild anaemia: 110–129 g/L
• Moderate anaemia: 80–109 g/L
• Severe anaemia: below 80 g/L

When treatment works, haemoglobin rises, and normal or near-normal haemoglobin means better energy, fewer transfusions, and fewer complications that can arise from repeated transfusions, like iron overload.¹

LDH (Lactate Dehydrogenase)

It is a simple blood test that goes up when red blood cells break apart. LDH is one of the fastest and easiest markers to check. Results often come around in a few hours. It is found in many tissues such as the liver, heart, brain, muscle, and red blood cells.
Two forms, LDH-1 and LDH-2, come mainly from RBCs. High LDH indicates ongoing haemolysis, which also means severe disease.

In untreated PNH, LDH can be over 10 times normal. A drop close to normal means treatment is working.

Persistently high LDH means ongoing haemolysis, which raises the risk of complications like blood clots.^1,5

Limitations: 

• LDH can rise for many reasons other than PNH (e.g., infections, liver disease, heart problems, muscle disorders, and vitamin deficiencies) 
• It can also be influenced by how the blood sample was collected or the type of machine used for testing 
• LDH also clears from the blood within about two days, so that levels can change quickly¹

Bilirubin

Bilirubin is made when haemoglobin is broken down. When bilirubin levels are high, it usually indicates haemolysis.  Untreated people often have raised levels, which can cause jaundice (yellow eyes/skin). Bilirubin is a useful marker because it reacts quickly to changes, such as if treatment is working and red cell destruction stops, it returns to normal very quickly (within about 4 hours).^1,5

Haptoglobin

Haptoglobin is a protein in the blood that binds free haemoglobin when red blood cells break apart. If haptoglobin levels are very low or completely absent, it means RBCs are bursting and spilling haemoglobin in the bloodstream.  This makes it a sensitive marker of haemolysis.^1,5

Limitations: 

• Even with good treatment, haptoglobin can stay low because small amounts of ongoing haemolysis can keep it down. That’s why it’s not as reliable
• Another protein, haemopexin, can give extra information, after haptoglobin is depleted, but it is not routinely measured in most clinics¹

Reticulocyte count

Reticulocytes are “baby” red blood cells freshly released from bone marrow. They usually increase because the bone marrow is working harder to replace destroyed cells. Reticulocytes help monitor how well the marrow is coping and whether treatment is helping. It can be calculated in any laboratory and is recommended to be monitored regularly in those with PNH.

• High count: Bone marrow is working overtime to replace destroyed cells (continued haemolysis, spleen overactivity, or immune reactions)
• Low count: Bone marrow is failing, or the body lacks nutrients like iron, B12, or folate

Reticulocyte counts normalise more slowly than other markers, usually taking days.^1,5

Basic Metabolic Panel (BMP)

BMP is a blood test that helps check kidney function and electrolytes. In PNH, kidney damage is common.  Doctors look for high creatinine (a waste product from muscle) and BUN (blood urea nitrogen, another waste product filtered by the kidneys). High levels may suggest chronic kidney disease.⁶

Urinalysis (UA)

A simple urine test to detect signs of broken red blood cells escaping into urine (haemoglobinuria), or leftover iron deposits (haemosiderosis), which can lead to pink/red urine.^1,5

Complete Blood Count (CBC)

This test measures RBCs, WBCs(White blood cells), and platelets
In PNH, it can show :

• Anaemia (low RBCs).
• Leukopenia (low WBCs).
• Thrombocytopenia (low platelets).

These changes are common when the bone marrow is weak.⁶

Flow cytometry

Flow cytometry is the most reliable test for diagnosing and monitoring PNH.⁵ It shows the percentage of blood cells that are affected (clone size). Larger clone size usually means more severe disease, stronger symptoms, more haemolysis and higher risk of clots.⁴

How it works:

The test uses special dyes or labels (monoclonal antibodies or a dye called FLAER) that stick to protective proteins. If the labels don’t attach, it shows those cells are missing their protection and are PNH cells.⁵ It is done mainly on WBCs (like neutrophils and monocytes) and RBCs. Checking the percentage of WBCs that are affected gives the best idea of the advancement of the disease. If the defect is found in the WBCs, doctors usually also check the red blood cells for confirmation. RBCs in PNH usually have a much shorter life than the usual 120-day lifespan.²

There are two versions of this test:

• Standard (low sensitivity) flow cytometry: Good enough to diagnose most cases of PNH⁶
• High-sensitivity flow cytometry: It is much more detailed.  It can detect even a very tiny number of affected cells, as little as 1% which is seen in more than 40% of the individuals^2,3

It’s checked every 6 months for the first 2 years after diagnosis, then once a year if stable and more often if symptoms change.^1–3

Other Specialised Tests

If flow cytometry shows severe disease or if complications are suspected, your doctor may order additional tests, which include:

• D-dimer: To check for blood clots
• BNP (brain natriuretic peptide): To check heart strain
• Liver and iron panels: To see if these organs are stressed or overloaded
• Bone marrow biopsy and genetic studies: To check marrow health and rule out related conditions⁶

Imaging Tests

Imaging helps find hidden complications:

• Echocardiography: To look for pulmonary hypertension
• Doppler ultrasound of the abdomen: To check blood flow in the liver and to detect clots
• CT pulmonary angiography: It is used if a lung blood clot (pulmonary embolism) is suspected
• CT of the abdomen: This is helpful if clots in liver veins (Budd–Chiari syndrome) are suspected
• MRI of the brain: It is used if clots in the brain veins are suspected⁶

Summary

Paroxysmal nocturnal haemoglobinuria (PNH) is a rare blood disease that people develop later in life. It happens when a random genetic change in the bone marrow makes blood cells that are missing important protective proteins (GPI-anchored complement regulatory proteins like CD55 and CD59). These proteins are normally attached to the cell surface by GPI anchors. When the anchors don’t work, the proteins fall off, and red blood cells are left unprotected. As a result, the body’s own defence system destroys them.

The main problem in PNH is the ongoing breakdown of red blood cells (haemolysis). Another serious risk is dangerous blood clots. Because symptoms can look very different from person to person, diagnosis is often delayed.

Lab tests are essential because they confirm the diagnosis, show if treatment is working, and help predict future risks.  Routine tests include: haemoglobin (to check for anemia), LDH, bilirubin, and haptoglobin (to measure haemolysis), reticulocyte count (to show how hard the bone marrow is working), basic metabolic panel or BMP (to assess kidney function), urinalysis (to detect blood breakdown products in urine), and a complete blood count or CBC (to give an overall picture of RBCs, WBCs, and platelets).

Flow cytometry is the most reliable test for diagnosing and monitoring PNH. It measures the percentage of defective blood cells (“clone size”). A larger clone size usually means more severe disease, stronger symptoms, more haemolysis, and a higher risk of clots.

If the disease is severe, doctors may order more tests (like D-dimer, BNP, liver/iron studies, or bone marrow biopsy) and imaging scans (echocardiography, abdominal ultrasound, CT, or MRI) to look for hidden clots or organ damage.