CDS Index Basis Trade: Harvesting Spread Dislocations

Key Takeaways

• Basis equals the index spread minus the PV01-weighted average of its single-name constituents; positive basis means the index is wide.
• The index is more liquid than the single names, causing it to overshoot in stress and creating the basis opportunity.
• Execution costs of crossing 125 bid-ask spreads on single names can equal or exceed the expected basis gain for small dislocations.
• Post-2009 Big Bang, CDX IG uses no-restructuring terms while some single names use Mod-R, creating a permanent structural basis.

What It Is

The basis is the difference between the traded spread of a CDS index, such as CDX.NA.IG or iTraxx Europe Main, and its intrinsic spread. The intrinsic spread is the PV01-weighted average of the CDS spreads on each of the index constituents.

Basis = Index spread - Intrinsic spread

Intrinsic spread = sum over i of (PV01_i x Spread_i) / sum of PV01_i

When the basis is positive, the index trades wide of its constituents. When negative, the index trades tight. A basis trade sells protection on the cheaper leg and buys protection on the richer leg, aiming for convergence.

The Intuition

The index and its constituents represent the same basket of default risk, so in theory their spreads should match. In practice they diverge because they are different products with different liquidity.

Index contracts are the most liquid credit instrument available, trading tight bid-ask with deep volumes. Single-name CDS are less liquid and adjust more slowly to news. When credit volatility spikes, the index overshoots the single names and the basis widens. When volatility fades, the basis compresses back toward zero.

The trade also picks up a structural premium. Single-name CDS settle on restructuring events, while index contracts since the 2009 ISDA Big Bang use "no restructuring" (XR) terms in North America. That difference means the index technically offers less protection, so it should trade slightly tighter than the constituents on average.

How It Works

The classic positive-basis trade buys protection on each of the 125 names in iTraxx Main or 125 names in CDX IG, and sells protection on the index for the same notional. A negative-basis trade does the opposite.

DV01 weights matter. Each single-name leg is scaled so that its PV01 matches its weight in the index (typically 1/125 = 0.8 percent). The trader funds the spread differential on day one and waits for convergence.

Execution cost is the main barrier. Crossing bid-ask on 125 single names can eat 2 to 4 basis points of the expected basis, so the trade only works when the mispricing is large enough. During the 2008 crisis, the CDX IG basis blew out past 100 bps as single names lagged the index, creating a generational opportunity for dealers who could warehouse the risk.

Worked Example

Assume CDX.NA.IG 5-year trades at 85 bps and its 125 constituents have a PV01-weighted average spread of 78 bps. The basis is +7 bps, meaning the index is 7 bps wide of fair value.

A trader takes a positive-basis convergence trade. They buy single-name protection on all 125 names at 78 bps average and sell index protection at 85 bps. Both legs size to 100 million notional in aggregate.

The position earns 7 bps per year in carry, or roughly 70,000 dollars on 100 million. If the basis compresses to 0 over six months, the mark-to-market gain on the spread differential adds another 7 bps times the 5-year PV01 (about 4.5), or roughly 315,000 dollars. Total profit before execution costs: 350,000 dollars on 100 million gross notional.

The risk is that the basis widens further. During the 2008 collapse, a trader carrying this exact position would have watched the basis double before snapping back months later.

Common Mistakes

• Ignoring restructuring clause differences. Post-Big-Bang, CDX IG uses "no restructuring" while single-name CDS may use Mod-R. A restructuring event pays the single-name leg but not the index, creating a structural basis the trade should not try to arbitrage away.
• Underestimating execution drag. Legging into 125 single names through multiple dealers can cost 3 to 5 bps of slippage on entry and another 3 to 5 on exit. A 5 bp basis rarely covers this.
• Holding through volatility spikes. Convergence trades bleed when correlation rises and the index leads. In 2008 and March 2020, the basis widened dramatically before it compressed. Risk limits must size for tail moves.
• Using stale constituent lists. Index rolls every six months replace defaulted or downgraded names. A basis calculation against the old on-the-run list after a roll gives the wrong answer.
• Forgetting funding costs. Dealers finance the single-name leg on repo or balance sheet. When funding spreads widen, the effective basis required for profit rises.

Frequently Asked Questions

What causes the CDS index basis to widen suddenly? The basis typically widens when credit markets become volatile or stressed. During stress, investors buy index protection rapidly because it is immediately available and liquid, pushing the index spread wider. Single-name CDS markets adjust more slowly because they are less liquid and require individual counterparty negotiation. The result is an index that temporarily overshoots the fair value implied by its constituents.

Why is the basis not a pure arbitrage opportunity? Pure arbitrage requires costless, riskless execution. The basis trade faces transaction costs from crossing 125 bid-ask spreads, funding costs, roll risk when the index refreshes, basis widening risk if stress continues, and counterparty risk. Additionally, the restructuring clause difference between the index and some single names creates a permanent structural component that should not necessarily close. These factors mean the trade carries real risk and requires sufficient mispricing to be economically viable.

How does the index roll affect the basis trade? CDS indices roll every six months to a new series with updated constituents (removing defaulted or heavily downgraded names and adding new qualifying names). A position that was calibrated against the old on-the-run series becomes off-the-run after the roll, and the basis calculation against the new series will differ. Traders must decide whether to continue holding the off-the-run basis or roll their position to match the new on-the-run index.

Can this trade be done with only the investment-grade index or also high-yield? Both CDX.NA.IG (investment grade) and CDX.NA.HY (high yield) indices have active basis trades. HY basis trades tend to have wider bases because single-name HY CDS is less liquid than IG and adjusts more slowly. However, execution costs in HY are also higher, so the break-even basis needed to profit is larger. The structural dynamics are the same but the magnitudes differ.

What happened to the CDS index basis in March 2020? During the COVID-19 market shock in March 2020, the CDX IG basis blew out significantly as investors rushed to buy index protection much faster than single-name markets could price in the same risk. This created one of the largest positive-basis opportunities in years. Convergence eventually occurred when volatility settled and single-name markets caught up, rewarding traders who had warehoused the position through the peak stress period.

Sources

1. S&P Dow Jones Indices. iTraxx / CDX Total and Excess Return Indices Methodology. https://www.spglobal.com/spdji/en/documents/methodologies/methodology-itraxx-cdx-tr-er-return-indices.pdf
2. S&P Dow Jones Indices. CDX: Tradable CDS Indices. https://www.spglobal.com/spdji/en/landing/topic/cdx-tradable-cds-indices/
3. ISDA. 2009 Big Bang Protocol. https://www.isda.org/traditional-protocol/big-bang-protocol/
4. Bocconi Students Investment Club. Trading Credit Derivatives. https://bsic.it/trading-credit-derivatives/

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.