When and why do Z-spread and ASW spread diverge, and what does the divergence tell an analyst?
Both Z-spread and ASW spread measure credit compensation, but they can give different numbers for the same bond. My CFA study group debated which is 'correct.' When do they diverge, and what causes the difference?
Z-spread and ASW spread measure credit compensation using different benchmarks and methodologies, so they can produce different values for the same bond. Understanding when and why they diverge is essential for fixed income relative value analysis.
Quick Definitions:
- Z-spread: Constant spread added to each government spot rate that makes the bond's discounted cash flows equal its market price
- ASW spread: Spread over SOFR received when buying the bond and entering a par asset swap
Sources of Divergence:
1. Swap Spread (Largest Driver):
The swap rate includes bank credit risk; government rates do not.
Z-spread (approx) = ASW spread + Swap spread
If the swap spread is 30 bps:
- Z-spread = 150 bps
- ASW spread = 120 bps
- Difference explained by the 30 bps swap spread
2. Bond Price Away From Par:
ASW spreads are sensitive to whether the bond trades at a premium or discount.
| Bond Price | ASW Spread Effect | Z-Spread Effect |
|---|---|---|
| At par (100) | No adjustment | No adjustment |
| Premium (105) | ASW spread is higher | No effect |
| Discount (95) | ASW spread is lower | No effect |
This happens because the asset swap is structured at par, and the premium/discount creates a PV mismatch that gets absorbed into the spread.
3. Shape of the Yield Curve:
Z-spread uses spot rates (which vary by maturity), while ASW spread uses a single swap rate. In a steep curve environment, these benchmarks diverge more.
Practical Example:
Goldcrest Energy 7-year bond, 4.5% coupon, priced at 93:
| Measure | Value | Benchmark |
|---|---|---|
| Z-spread | 185 bps | Government spot curve |
| ASW spread | 148 bps | SOFR (via swap) |
| Swap spread (7yr) | 28 bps | |
| Par adjustment | ~9 bps | |
| Total divergence | 37 bps | 28 + 9 = 37 bps |
What the Divergence Tells You:
Trading Signal:
If Z-spread and ASW spread diverge beyond what swap spreads and par adjustments explain, it suggests a potential mispricing. An analyst can:
- Calculate the CDS-bond basis (CDS spread minus ASW spread)
- If the basis is unusually wide, consider a basis trade
- Compare the bond's Z-spread to rating-matched peers for relative value
Which Spread to Use When:
| Context | Preferred Spread |
|---|---|
| Buy-side analysis | Z-spread (clean credit view) |
| Bank trading desk | ASW spread (reflects funding cost) |
| Basis trading | ASW spread (direct comparison to CDS) |
| Cross-market comparison | Z-spread (removes bank credit noise) |
CFA Exam Focus: Know both spread definitions, understand the swap spread link, and recognize when par/off-par adjustments matter. Expect calculation questions with one spread given and the other derived.
Practice spread analysis in our CFA fixed income question bank.
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