The U.S. Food and Drug Administration flagged quality issues at privately held laboratory operator Theranos' manufacturing plant in Newark, California.
Theranos' main product is the Capillary Tube Nanotainer, one of two kinds of tiny vials used to collect blood from patients.
The company, which has a valuation of $9 billion, promises to shake up medical testing by conducting a wide range of tests with one drop of blood from a finger-stick using its Nanotainers, rather than the large vial typically collected.
The FDA said on Tuesday quality audits had not been performed at the Newark facility and that the company failed to document internal quality audit schedule.
The company was not immediately available for comment.
Theranos has been in the news since the Wall Street Journal published a pair of stories suggesting the startup was relying on tools from traditional labs as it struggled with its own technology.
Chief Executive Elizabeth Holmes, a Silicon Valley darling, fought back strongly, saying her company's technology was sound.
The healthcare startup has obtained FDA nod to sell blood tests for the notorious sexually transmitted herpes simplex virus. Theranos has applied for FDA approval for other tests.
The company, which was not immediately available for comment, has the option of discussing either its objection to the regulator's observations or corrective actions with FDA representatives.
The FDA, which inspected Theranos' Newark facility on Sept. 16, made its observations in a form 483, a document that the regulator issues if it finds violations of federal manufacturing guidelines.