Virginia's accounting system for its $120 million mental hospital program is unable to determine the daily amount of unpaid bills at hospitals, total individual patient charges or what losses should be written off because of insurance settlements.

That assessment is contained in a report issued last week by Auditor of Public Accounts Charles K. Trible.

The report goes on to say that "the system . . . does not provide . . . an effective means . . . to insure that all possible monies are collected."

One auditor agreed in an interview that the state's 16 mental health and mental retardation hospitals can be compared to a department store chain that keeps no record of unpaid credit purchases.

Walter J. Kucharski, the state auditor responsbile for mental health agencies and Hoover B. Lide, director of reimburesements for the state Department of Mental Health and Mental Retardation, said they cannot estimate losses that have resulted from the department's accounting practices.

"No one knows what has been lost if anything has been lost," Kucharski said. Each declined to label the difference between hospital charges and collections as "losses" because the state and federal governments make up the difference through government insurance and mental helath appropriations.

Lide said the department is in the final stage of implementing a computerized accounting system that, for the first time, is beginning to provide a record of unpaid bills and an evaluation of collection efforts.

The system has taken more than six years to develop. "There have been problems all along the line," Lide said. "When you have an accounting system, it is relatively easy to adapt it to data processing. But when we began, we didn't really have an accounting system."

In accounting terms, Tribe has criticized the department for failing to maintain a record of "accounts receivable," which simply is a record of unpaid bills.

"We had to jump ahead many years in time to set up an accounts receivable system," Lide said. He said that until recently the state did not "set up charges in advance of patient payments."

In the past the only record of total charges to a patient were in patient record jackets kept at hospitals. These files were simply kept in drawers and were susceptible to misplacement, theft and fires. "Anything could happen to them," Kucharski said.

The department's central reimbursement office has kept a record of payments by patients, their families, private insurance companies and government insurance, but has never been able to compare these to the total charges due.

The most recent criticism of the hospital accounting system was contained in an audit of the Northern Virginia Mental Health Institute released last week. Comments in it criticizing the accounting system were almost identical to those in last year's audit of the Fairfax County facility and in a 1976 audit of the Department of Mental Health and Mental Retardation, its parent agency.