Lalu Yadav, who was serving his jail term in Ranchi, was referred to AIIMS by the Rajendra Institute of Medical Science (RIMS) in Ranchi.
"His condition is stable. He was brought in at noon. His sugar levels are a little bit high and he has some infection in the kidney," an AIIMS doctor said.
He also said that Lalu Yadav has been admitted under the medicine department.
The RJD chief had been admitted to RIMS in Ranchi on March 17 over complaints of uneasiness while he was lodged at the Birsa Munda Jail. Yadav has been serving a jail term since December 23.
He left for New Delhi yesterday by train after a special CBI court allowed him to seek treatment at Delhi AIIMS.
"His condition was not suitable for flying and the doctors at RIMS advised him to travel by train," Mr Sinha said.
A medical board at the RIMS where Lalu Yadav was being treated had decided to refer him to AIIMS for better treatment, RIMS Director RK Srivastav said.
The former chief minister has been convicted in four fodder scam cases since 2013 -- the latest being the Dumka treasury case, in which a special CBI court sentenced him to 14 years in jail.
The Jharkhand High Court will hear on April 6 his bail plea in connection with his conviction in the third fodder scam case -- pertaining to fraudulent withdrawal of Rs 37.62 crore from the Chaibasa Treasury, Mr Sinha said.
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