If you're planning on applying for a mortgage, make sure you have your act together. Getting credit to buy a home is becoming more of a challenge.

The latest credit availability index (HCAI) released by the Housing Policy Finance Center indicates that lenders are becoming more choosy about what type of customer they will grant a mortgage to.

Although mortgage credit availability only took a small drop from 5.3 in the second quarter of 2015 to 5.0 in the third quarter, the decline in mortgage availability is part of a downward trend observed throughout 2015.

The HCAI is an important indicator of mortgage requirements because it measures what percentage of home purchase loans are likely to default. When a large part of mortgaged homes are likely to default, lenders apply more stringent criteria to mortgage applications, which makes getting a mortgages more difficult.

But the HCAI isn't the only index showing this trend. The Mortgage Credit Availability Index (MCAI) paints a similar picture, and indicates that the decrease in mortgage availability was still present in December 2015. This report published by the Mortgage Bankers Association (MBA), use Ellie Mae's AllRegs® Market Clarity® business information tool data to gauge credit availability for home loans.

In December 2015, the MCAI decreased 2.4 percent to 124.3 points. The index uses March 2012 as a benchmark at 100 points.

But MBA's Vice President of Research and Economics, Lynn Fisher, says that the December results are not the effect of a tightening of credit on the part of lenders, which would normally be the case. According to Fisher, the decrease was largely the result of technical issues related to the implementation of affordable loan programs with a low down payment.