The ratings of six medical devices stocks are down this week, according to the Portfolio Grader database. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

Antares Pharma, Inc.’s (ATRS) rating falls this week to an F (“strong sell”), down from last week’s D (“sell”). Antares Pharma develops pharmaceutical delivery systems, including needle-free and mini-needle injector systems and transdermal gel technologies. In Portfolio Grader’s specific subcategories of Equity and Cash Flow, ATRS also gets F’s. For more information, get Portfolio Grader’s complete analysis of ATRS stock.

Cardiovascular Systems, Inc. (CSII) is on the decline this week, earning a D (“sell”) after receiving a C (“hold”) last week. Cardiovascular Systems focuses on developing and commercializing minimally invasive treatment solutions for vascular disease. The stock also gets an F in Equity. To get an in-depth look at CSII, get Portfolio Grader’s complete analysis of CSII stock.

This week, Masimo Corporation’s (MASI) rating worsens to a D from the company’s C rating a week ago. Masimo designs, develops and licenses medical signal processing and sensor technology for the noninvasive monitoring of physiological parameters. For more information, get Portfolio Grader’s complete analysis of MASI stock.

Tandem Diabetes Care, Inc. (TNDM) earns an F this week, falling from last week’s grade of D. The stock gets F’s in Equity and Cash Flow. To get an in-depth look at TNDM, get Portfolio Grader’s complete analysis of TNDM stock.

Derma Sciences, Inc. (DSCI) earns a D this week, moving down from last week’s grade of C. Derma Sciences markets and sells a range of skin care, wound management, and specialty securement devices. The stock gets F’s in Equity, Cash Flow and Sales Growth. For more information, get Portfolio Grader’s complete analysis of DSCI stock.

Accuray Incorporated’s (ARAY) rating weakens this week, dropping to a D versus last week’s C. Accuray designs, develops, and sells medical radiation systems for the treatment of tumors anywhere in the body. The stock gets F’s in Earnings Momentum, Earnings Revisions and Equity. As of Dec. 24, 2014, 19.6% of outstanding Accuray Incorporated shares were held short. To get an in-depth look at ARAY, get Portfolio Grader’s complete analysis of ARAY stock.

Louis Navellier’s proprietary Portfolio Grader stock ranking system assesses roughly 5,000 companies every week based on a number of fundamental and quantitative measures. Stocks are given a letter grade based on their results — with A being “strong buy,” and F being “strong sell.” Explore the tool here.