Adds graphic, updates share moves in paragraphs 2-3

By Rajasik Mukherjee

May 11 (Reuters) - Australian biotech firm CSL CSL.AX slashed its fiscal 2026 outlook on Monday and flagged a $5 billion impairment charge, sending its shares to a near decade-low.

Shares of CSL, once Australia's priciest stock, fell as much as 21.9% to A$93.640, their lowest since mid-December 2016, and logged their worst trading day on record.

The stock has lost over 40% in value this year, sharply underperforming the top 20 firms in Australia .ATLI, which is trading about 4.5% in green.

CSL’s prolonged underperformance reflects a series of confidence‑shaking blows over the past year, including a Pentagon decision to drop a seasonal flu vaccine mandate for troops, the abrupt exit of its chief executive, and growing uncertainty around the future of its vaccine arm spin‑off.

Interim CEO Gordon Naylor said he had conducted a review of the business after taking on the role in February and the financial benefits from growth initiatives had yet to materialise.

CSL, a former government laboratory that became a stock market darling, said it expected to recognise about $5 billion in non‑cash, pre‑tax asset impairments across fiscal years 2026 and 2027, including writedowns of CSL Vifor intangible assets such as parts of its product portfolio.

The firm had shelled out about $12 billion for the acquisition of Vifor Pharma in 2022.

For 2026, CSL now expected a net profit after tax attributable, excluding restructuring costs and impairments, of about $3.1 billion on a constant-currency basis, compared with $3.3 billion reported a year earlier.

"On the guidance cut - this is the second trim in roughly six months, and that pattern matters," said Mark Gardner, founder and CEO of MPC Markets, a Sydney-based investment advisory firm.

"Back-to-back downgrades do raise legitimate questions about the company's visibility into its own business."

In August last year, the firm projected its 2026 NPATA to be in the range of $3.45 billion to $3.55 billion, representing growth of about 7%-10%.

On Monday, the company cut its fiscal 2026 revenue forecast to $15.2 billion on a constant-currency basis, compared with $15.6 billion reported last year.

CSL said its revised revenue outlook reflected an estimated $300 million hit to revenue from a buildup in immunoglobulin inventory in the U.S., about $200 million from lower albumin market value in China, and roughly $150 million from the impact of the Middle East conflict and other headwinds.

"The broader picture is a firm in the middle of a complex transformation - still without a permanent CEO, dealing with the fallout of an overpriced acquisition, and navigating real revenue headwinds in its two biggest markets," Gardner said.

Last year, in October, the company cut its NPATA forecast to 4%-7% growth and its 2026 revenue guidance to 2% to 3%, down from a 4% to 5% range.