Monday, December 7, 2009

Consistent Time

Guest author and Vice Chair of the HIT Standards Committee John Halamka reports on HITSP's Consistent Time Transaction and its implications for clinical decision making and health information exchange.

Consistent Time
by John Halamka

I was recently asked by my staff how we should coordinate the time of day across organizations which exchange healthcare information. In a future which treats data from outside data sources as appropriate for clinical decisionmaking, you can imagine the following data exchange:

Hospital 1 posts lab result 12:01pm
Hospital 1 sends result to Hospital 2 12:02pm
Hospital 1 revises lab result 12:15pm
Hospital 1 sends revision to Hospital 2 12:16pm
Order is entered at hospital 2 12:17pm

Time synchronization among participants in a healthcare information exchange is important. If Hospital 2's clocks were 3 minutes slow, it would be challenging to know if the order was entered based on the original or revised lab result.

HITSP has published T16, the Consistent Time Transaction to help address this problem. It's based on an IHE profile created to support the synchronization of security audit logs.

Here is the relevant section IHE ITI TF Vol 2: from IHE-CT profile
"The NTP transactions are described in detail in RFC1305. There is also extensive documentation on the transactions and recommendations on configurations and setup provided at Rather than reproduce all of that material as part of this Framework, readers are strongly encouraged to explore that site. The most common mode is the query-response mode that is described below. For other forms, see RFC1305 and the material on

The Time Server shall support NTP (which implicitly means that SNTP clients are also supported). Secure NTP may also be supported. The Time Client shall utilize NTP when it is grouped with a Time Server, or when high accuracy is required. For ungrouped Time Clients with 1 second accuracy requirements, SNTP may be useable. Time Clients may also support Secure NTP."

Although original designed for audit trails, the transaction has been expanded to other transactions, since organizations have realized that having synchronized clocks really helps documentation integrity and workflows. As the use of the consistent time is extended beyond audit trails, there are interesting issues about just how precisely synchronized devices in a network should be - a few seconds, one second, a subsecond?

At BIDMC, we point to stratum 1 servers that are directly connected to computers attached to atomic clocks.

The interesting question for HIEs is what should be synchronized.

My hospital servers are all synchronized against one set of time sources.

Our HIE, NEHEN, has suggested that all the gateways used to exchange data among multiple hospitals should be synchronized with one time source to ensure that all send and receive timestamps for clinical data exchange are consistent. Otherwise, data might arrive at one hospital before it leaves another!

However, since HIE gateways will be synchronized with one time source and hospital internal servers may be synchronized with others, the HIE time may vary from the hospital time.

Maybe the right answer is that as part of our national healthcare IT effort, we should mandate that all hospitals and HIEs should use a single set of known-adequate time servers to ensure all healthcare time is consistent.

For the moment, the following strategy seems reasonable

1. Require hospitals use NTP to ensure their internal time stamps are consistent. This will ensure audit trails within an organization, whether merged in an audit repository or just reported from disparate systems upon request, are consistent.

2. Synchronize health information exchange gateways within an HIE to a single time source so that transactions have consistent send and receive times.

If we know the hospital audit trail time stamp is consistent and we know the HIE send/receive times are consistent, we can recreate any event that is disputed.

Expecting every hospital to change its time synchronization servers to those used by the HIE is unrealistic - what if the hospital participates in multiple HIEs?

At some future time, we all may change to a national healthcare time server that is part of the NHIN, but for now the hospital use of NTP will be decoupled from the HIE use of NTP.

No comments: