Injection of an irritant, such as formalin into the skin will result in the persistent activation of small sensory afferents and will evoke an organized constellation of responses which includes favoring, licking and flinching of the injected paw. The incidences of these nocisponsive behaviors occur in two phases: an initial first phase (phase 1, 0-10 min) and a delayed second phase (phase 2, 10-60 min).
The formalin test is a complex model that probes the pharmacology and physiology of systems that are activated by tissue injury. The formalin test is a valid model for components of clinical pain associated with tissue injury. This places it in contrast to other models where the response is evoked by an acute, transient stimulus that does not produce tissue injury.
The formalin test is in wide use for mechanistic studies of nociception and for the evaluation of analgesic and anti-hyperalgesic agents in rats and mice.
The test involves injecting one hind paw of the rodent with a small volume (10-50µL) of formalin (1-5%). The animal is observed to favor the paw and display periodic flinches in the injected limb. These flinches are counted at periodic intervals for typically up to an hour after injection. In normal practice, the observer is required to discriminate between a response or normal movement of the animal.
Relying on human observers, the test requires considerable training to establish high “inter-observer” reliability; it is tedious, requiring uninterrupted attention on the part of the observer and finally it is labor intensive.
The ANA device was designed to address these issues. ANA automatically detect the occurrence of paw flinches. This is accomplished by measuring the movement of a small metal band (0.5 grams) that is placed on the injected paw; the animal is placed without restraint inside the observation chamber over an electromagnetic detector system. 
Paw flinches are detected by the system and counted automatically using a computer. At the end of the test, a file is written that contains the comment for each observed animal and the number of flinches per minute over time. The base system can accommodate 4 animals at a time. The upgrade package will allow the researcher to measure an additional 4 animals concurrently.
Depending upon study parameters this system can thus permit a single technician to undertake relatively large scale screening (6 runs/day x 8 rats/run). Moreover, the design of the system reduces the time required to train a technician. The time and labor to develop validating results is reduced to essentially a day.
The system as configured includes equipment and supplies to test 4 animals simultaneously and allow 4 more animals to acclimate over the period of testing. The equipment consists of, 4 detection devices, 8 clear plastic observation cylinders, signal acquisition module and BNC input interface with interconnecting cable, automated formalin testing, and data analysis software.
Included with the analysis software, is a program for searching standardized file names, and moving data from these files into an Excel spreadsheet. (template supplied) The Excel spreadsheet calculates the count, mean, standard deviation, standard error and % maximum possible effect (MPE), on the control, drug, and dose data.
The source documentation is at: