A new report from Citigroup says that a new bubble is on the horizon, but the sector may surprise you. The report points to health care as the latest to see troubling signs.

The two economists at Citigroup who wrote the report, Steven Wieting and Shawn Snyder, reason that "functioning market price competition barely exists" in health care, while spending has grown 2.5 times the rate of income over the last 30 years. Perhaps the cause is the fact that patients only finance around 12% of their health care expenditures in the U.S., with the rest covered by private insurance and Medicare -- much like the public/private partnership of the housing finance bubble (via CNBC).

If the cost is shifted more to patients, demand for the pricier treatments may collapse. Short of that, "nearly unlimited taxpayer resources to fund programs like Medicare has economists increasingly concerned," according to CNBC.

Business section: Investing ideas
Do you think health care spending is out of control? Is the sector likely to see a big correction?

For a closer look at the largest health care names, we ran a screen on large-cap health care providers traded in on the U.S. exchanges. Do you think they're at a tipping point? (Click here to access free, interactive tools to analyze these ideas.)

1. Aetna (NYSE: AET): Operates as a diversified health care benefits company in the United States. It has a market cap of $17.39 billion and a stock price of $48.82.

2. Cigna: Operates as a health service organization. It has a market cap of $14.08 billion and a stock price of $48.02.

3. Express Scripts (Nasdaq: ESRX): Provides a range of pharmacy benefit management services in North America. It has a market cap of $29.96 billion and a stock price of $56.44.

4. Humana (NYSE: HUM): Offers various health and supplemental benefit plans in the United States. It has a market cap of $14.96 billion and a stock price of $89.39.

5. Unitedhealth Group (NYSE: UNH): Provides health care services in the United States. It has a market cap of $61.66 billion and a stock price of $58.24.

6. WellPoint (NYSE: WLP): Operates as a health benefits company in the United States. It has a market cap of $24.07 billion and a stock price of $70.61.

Interactive Chart: Press Play to compare changes in analyst ratings over the last two years for the stocks mentioned above. Analyst ratings sourced from Zacks Investment Research.