One challenge in creating such a device
is in understanding its accuracy, as illustrated in the following practical example.
[ The Centice PASS Rx smart sensor ]
Prescription verification
Pharmacists face ever-increasing demands
to process more prescriptions without
sacrificing accuracy. As the population ages, the number of prescriptions
dispensed in the United States is growing
rapidly, estimated at 3. 9 billion in 2009.
Manual prescription verifications (visual
inspections) are subjective and therefore
prone to error. According to a 2003 study
published in the Journal of the American
Pharmacists Association, the overall error
rate for dispensing pharmaceutical medications was an alarming 1. 7 percent.
Prescription errors can have devastating consequences. According to the
Institute of Medicine, at least 1. 5 million
Americans are injured by medication
mistakes each year. Raman spectroscopy
offers a solution: It can provide pharmacists with a quantitative verification
process while prescriptions are being dispensed. However, pharmacists will only
adopt a sensor that fits seamlessly into
their workflow, covers their workload
sufficiently, adapts to the ever-changing
landscape of available drugs, and, most
important, performs accurately. It is
vital, therefore, for the engineer to understand the performance tradeoffs of the
sensor system.
A company called Centice Corp. has
developed its “PASS Rx” system, which
uses Raman spectroscopy to verify that
the pills in a bottle match the prescription label. The pharmacist scans the prescription’s bar code and places the bottle
into the PASS Rx device. The device
positions the bottle, acquires a spectrum,
analyzes the spectrum, and assesses
whether the contents of the bottle match
the prescription label. To answer this
question, the Raman spectrum is first
processed before applying the classifier to
make the final yes/no determination.
The processing steps help to remove
variation caused by the pill bottle’s
spectrum and fluorescence as well as
the differences caused by the size and
type of the container, the kind of pill,
1
Scan barcode
and place vial
in chamber.
2
Press the
verify button.
3
Read the
result.
The PASS Rx system uses Raman spectroscopy to verify the accuracy of pharmaceutical prescriptions.
140,000 120,000 100,000 80,000 60,000 40,000 20,00006008001,0001,2001,4001,6001,800
Ramen shift in cm- 1
Ramen shift in cm- 1
600 800 1,000 1,2001,400 1,6001,800
6,000
5,000
4,000
3,000
2,000
1,000
0
NIST correction
Normalization
Baseline correction
Bottle subtraction
The spectrum of lithium carbonate before and after calibration and processing.
and the quantity of medication. After
gathering and processing the spectra, the
classifier scores the measurement relative
to a threshold. If the score significantly
exceeds the threshold, the sensor may
confidently say that the drug inside the
bottle and the label match.
ROC curve analysis
Consider two experiments to measure
the performance of the PASS Rx system.
In the first, PASS Rx is repeatedly
tested with correctly dispensed prescrip-
tions. The classifier score for each test is
calculated and compared to a threshold
in order to assess whether there is a
dispense (fill) error. The second experi-
ment is similar; however, here the system
is tested exclusively with prescription
errors. (The actual error case experiments
are complicated by different manufactur-
ers and strengths for the same drug, but
these details are not considered here.)
Based on these two experiments, there
