The study population was 40,512 noninstitutionalized adults who took part in the National Epidemiologic Survey on Alcohol and Related Conditions and who in 2001-2002 (Wave 1) didn't meet criteria for lifetime mania or hypomania, but indicated the presence of elation and/or irritability. (The survey is sponsored by the National Institute on Alcohol Abuse and Alcoholism.) The researchers studied the same cohort in 2004-2005 and found that "The likelihood of developing a clear episode of mania or hypomania by Wave 2 was significantly increased in subjects with elation or only irritability at Wave 1, compared with subjects who did not endorse either. Endorsement of both symptoms at Wave 1 increased the likelihood of a new epsisode of mania or hypomania 4.6 times, which was significantly higher than for those with only elation or irritability."
Read more about recent research on bipolar disorder in Psychiatric News here and here.