The Food and Drug Administration (FDA) has just taken the unprecedented step of sending a letter to the developer of an iPhone medical app.

In its letter, the U.S. agency contends that it should’ve been consulted first before Biosense Technologies released the uChek app in the App Store.

Launched a month ago and optimized for iPhone 4, 4S and 5, uChek is designed for use with certain test strips or dipsticks in checking levels of blood, glucose and other substances in urine.

Biosense notes the app’s accuracy as follows:

The uChek urine analyzer has been tested extensively at two sites with approximately 800 samples with the iPhone 4, 4S and 5. uChek has been found to be accurate with 100% readings within +/- 1 color block of a urine dipstick. This makes the performance of uChek comparable with many commercially available semi-automated urinalysers.

But while it does not dispute the app’s accuracy per se, the FDA asserts in its letter to Biosense that the urinalysis dipsticks uChek is designed to analyze are only cleared for “direct visual reading”:

Since your app allows a mobile phone to analyze the dipsticks, the phone and device as a whole functions as an automated strip reader. When these dipsticks are read by an automated strip reader, the dipsticks require new clearance as part of the test system.

For that reason, the FDA urges Biosense to obtain clearance for its test system comprising uChek and the dipsticks compatible with it.

In a statement to Bloomberg, Biosense cofounder Abhishek Sen said:

We intend to work very closely with the U.S. FDA over the coming months to ensure that we continue to deliver accurate, affordable and convenient diagnostics across the world.

The uChek app remains available in the App Store for free. It’s compatible for use with the following urine strips: Siemens Multistix SG10, Siemens Multistix SG, Siemens Uristix, Bayer Keto-Diastix, and Bayer Diastix.