  The increasing complexity of algorithms for analyzing medical data, including
de-identification tasks, raises the possibility that complex algorithms are
learning not just the general representation of the problem, but specifics of
given individuals within the data. Modern legal frameworks specifically
prohibit the intentional or accidental distribution of patient data, but have
not addressed this potential avenue for leakage of such protected health
information. Modern deep learning algorithms have the highest potential of such
leakage due to complexity of the models. Recent research in the field has
highlighted such issues in non-medical data, but all analysis is likely to be
data and algorithm specific. We, therefore, chose to analyze a state-of-the-art
free-text de-identification algorithm based on LSTM (Long Short-Term Memory)
and its potential in encoding any individual in the training set. Using the
i2b2 Challenge Data, we trained, then analyzed the model to assess whether the
output of the LSTM, before the compression layer of the classifier, could be
used to estimate the membership of the training data. Furthermore, we used
different attacks including membership inference attack method to attack the
model. Results indicate that the attacks could not identify whether members of
the training data were distinguishable from non-members based on the model
output. This indicates that the model does not provide any strong evidence into
the identification of the individuals in the training data set and there is not
yet empirical evidence it is unsafe to distribute the model for general use.
