June 21 (Reuters) - The U.S. Food and Drug

Administration on Thursday approved Roche's drug for a

chronic blood disorder, the Swiss drugmaker said.

The drug crovalimab, branded as PiaSky, is a monthly

under-the-skin or intravenous treatment for paroxysmal nocturnal

hemoglobinuria (PNH).

PNH is a disorder in which red blood cells break apart

prematurely. It can cause anemia, fatigue and blood clots, and

can lead to kidney disease.

Roche said the disease affects around 20,000 people

worldwide.

The approval is based on a late-stage study in which PiaSky

showed a 79.3% control in the destruction of red blood cells

versus 79% for the standard-of-care eculizumab from week 5 to

week 25.

Other treatments for PNH such as Astrazeneca's ( AZN )

Ultomiris and eculizumab, sold as Soliris, and Amgen's ( AMGN )

Bkemv require infusion by healthcare professionals.

PiaSky was approved in China in February and Japan in March.

The drug is also being tested in two other blood disorders,

atypical hemolytic uremic syndrome and sickle cell disease, and

a kidney disease called lupus nephritis.