What happened Shares of Tandem Diabetes Care (NASDAQ: TNDM) were jumping 12.9% higher as of 3:51 p.m. EDT on Tuesday. The gain came after S&P Global announced that the stock would be added to the S&P MidCap 400 Index effective Sept. 20. So what There's one big advantage that mid-cap stocks obtain from being added to the S&P MidCap 400 Index. Any mutual fund or exchange-traded fund that tracks the index must add the stocks to their holdings. Image source: Getty Images. That's the main reason Tandem enjoyed a boost today. Investors liked the extra attention that the company will now receive from funds. However, any positive momentum for the stock from its addition to the MidCap 400 should only be temporary. Once funds have bought shares of Tandem, the catalyst will be over. Now what What really matters for Tandem is the sale of its insulin pumps. The good news for investors is that the company generated record sales in the second quarter thanks to rapid growth in the U.S. market. Even better, it is only in its early stages of penetrating the international market. 10 stocks we like better than Tandem Diabetes CareWhen our award-winning analyst team has a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* They just revealed what they believe are the ten best stocks for investors to buy right now... and Tandem Diabetes Care wasn't one of them! That's right -- they think these 10 stocks are even better buys. See the 10 stocks *Stock Advisor returns as of August 9, 2021 Keith Speights has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends S&P Global. The Motley Fool has a disclosure policy.Source