Wariness about leaving diagnosis to Google, however, is a valid response. For one, the PHQ-9 is never meant to be a diagnostic tool on its own. It's meant to be administered by a physician who can also professionally determine other things that could be causing depressive symptoms besides depression itself, like bereavement or bipolar disorder. Context, when it comes to mood disorders, is very important: Depression is an extremely complex disorder, comprising genetic vulnerability, physical health, past traumas, stress, addiction issues, co-existing mental health problems, and a host of other different possible contributing factors. The existence of the PHQ-9 is just a number on a page; to be truly effective, it has to be paired with in-person work with a professional.
"Other co-morbidities may exist (e.g. anxiety disorders), and the depression may be due to central nervous system disorders, endocrine disorders (e.g. hypothyroidism), medication or other medical disorders," Harvard psychiatrist Srini Pillay, M.D., tells Bustle. "Ignoring this may lead to a medical disorder being untreated. The depression screening may be misleading in that people may believe that they have an 'emotional' problem and ignore the medical causes as well."
The difficulty with the PHQ-9 and its results, when not administered by a physician, is that people who obtain a major depressive score may attempt to treat it by themselves, with St. John's Wort, exercise or meditation — which is an excellent idea unless the depression itself is caused by underlying neurochemical factors that can't be diagnosed without a doctor.
"The PHQ-9 is certainly a relevant tool as it relates to depression," Pillay notes, "but there are a few things to bear in mind." For one, he explains, "while it may be helpful as a screening tool for depression, the interpretation can be more complicated in certain populations such as the coronary artery disease population. Also, it is a rough measure of depression severity and it is not a diagnostic instrument."
The other factor to consider is that the PHQ-9, while widely accepted, isn't the only measuring tool for depressive symptoms. Another is called the QIDS-SR (the Quick Inventory of Depressive Symptomatology, Self-Report), which covers much the same ground but in different ways and with an altered scoring system. And the World Health Organisation has something called a World Mental Health Composite International Diagnostic Interview, which is meant to be administered by a trained interviewer to diagnose many kinds of mental illness, including co-existing ones. The PHQ-9 isn't the only option out there, and depression is sufficiently diverse that it may prove inaccurate for a minority of patients who do actually need help.
"It is important to bear in mind that online self-diagnosis is a weak data point, and that the best way to establish whether one has a clinical depression is to visit a professional such as a psychiatrist, psychologist or primary care physician," Pillay tells Bustle. "The internet is a valuable adjunctive resource, and it may be useful to take one’s own findings in to that visit with a professional."
Overall, Google's work is moving in the right direction — but don't take it as a diagnosis without consulting a professional and getting all the information you can get. When it comes to mental health, you can never be too careful.