Why Does PNH Cause Dark Urine in the Morning? Explained

If you have Paroxysmal Nocturnal Hemoglobinuria (PNH), your urine may become dark—sometimes looking like tea or cola—because red blood cells are breaking apart inside your blood vessels. This destruction releases a red protein called hemoglobin into your bloodstream, which your kidneys then filter into your urine, giving it a dark color ^[1][2]. The reason this discoloration is often most noticeable in the morning is largely because you go several hours without drinking fluids while you sleep, which concentrates the hemoglobin pigment ^[1]. However, dark urine is a non-specific clue; it is not a perfect measurement of your disease activity, and blood or urine tests are needed to confirm exactly what is causing the color change ^[3].

The Science Behind the Color

To understand why this happens, it helps to look at the mechanics of PNH. PNH is caused by an acquired genetic mutation (usually in the PIGA gene) that happens in your blood-forming stem cells during your lifetime ^[4][5]. This means it is not a condition you inherited from your parents, nor is it something you will pass to your children.

This mutation causes your body to produce red blood cells that lack specific protective proteins, known as CD55 and CD59, on their surfaces ^[4][5]. Without these shields, your red blood cells are vulnerable to being attacked by your immune system’s complement system. This leads to intravascular hemolysis—the medical term for red blood cells rupturing while they are circulating in your bloodstream ^[3][6].

When these cells break open, they release free hemoglobin into the blood plasma. The kidneys then filter this excess hemoglobin out of the body through the urine, a condition known as hemoglobinuria ^[3][1]. The dark color you see is the pigment of the hemoglobin itself, not necessarily whole, intact red blood cells. A urinalysis can help your care team distinguish between free hemoglobin and other causes of discoloration.

Why is it Called “Nocturnal”?

The “nocturnal” part of PNH is a historical leftover. Because patients frequently reported waking up and passing very dark urine in the morning, early physicians mistakenly believed the destruction of blood cells only happened at night.

Today, experts know that hemolysis can happen continuously throughout the day, and the timing of these events varies from person to person ^[4]. Morning urine usually appears darker simply because the pigment is highly concentrated after a night of sleep. When you drink fluids during the day, your urine becomes diluted and may look lighter, even though cell destruction might still be occurring ^[4].

What This Means For You

While dark urine can be a visible clue of hemoglobinuria, it is not a reliable home test to measure how well your PNH is controlled. Active hemolysis can happen without visibly dark urine, and dark urine can also be caused by dehydration, certain foods, liver issues, or other medications.

• If you are on treatment: Modern PNH therapies, such as complement inhibitors (like eculizumab), are designed to stop the immune system from destroying your red blood cells ^[7]. If you notice dark urine while on these medications, it could indicate “breakthrough hemolysis” (meaning the medication isn’t fully controlling the cell destruction) ^[7]. However, this can happen for many reasons, including a recent infection or a missed or delayed medication dose. Do not change or stop your treatment on your own. Your doctor will need to run lab tests—such as an LDH, reticulocyte count, bilirubin, and haptoglobin—to evaluate what is happening.
• Hydration: Follow your care team’s specific advice on daily fluid intake. While staying properly hydrated can help dilute your urine, forcing yourself to drink excess water will not stop the underlying cell destruction and can be unsafe if you have heart or kidney conditions.

When to Seek Emergency Care

Because PNH carries a risk of serious complications, including blood clots (thrombosis) and kidney injury, you should seek emergency medical attention if your dark urine is accompanied by any of the following “red flag” symptoms:

• Severe abdominal, chest, or back pain
• Shortness of breath or trouble breathing
• Sudden severe headache, fainting, or neurological changes
• Pain, redness, or swelling in one leg
• A significant drop in how much urine you are producing