Spread Duration
Spread duration measures the sensitivity of a bond's price or a portfolio's value to a one basis point change in the credit spread (the yield premium demanded by investors above the risk-free rate) rather than to changes in the absolute level of risk-free yields, providing a tool for assessing credit spread risk independently of interest rate risk.
In fixed income portfolio management, total yield risk decomposition requires separating two distinct sources of yield movement: changes in the risk-free benchmark rate (captured by modified duration or interest rate duration) and changes in the credit spread above that benchmark (captured by spread duration). For a single bond with no optionality, these two metrics are approximately equal and often treated interchangeably. The distinction becomes practically meaningful when managing portfolios with diverse credit quality, or when risk-free rates and credit spreads move in different directions — a common scenario during financial stress periods when risk-free government bond yields fall (flight to safety) while credit spreads widen simultaneously.
For the Indian corporate bond market, spread duration analysis has grown in importance as the market has developed and institutional participation has increased. A credit portfolio manager overseeing a mix of government securities, PSU bonds, AAA corporate bonds, and AA bonds must distinguish between the portfolio's pure interest rate risk (DV01 measured against the G-Sec yield curve) and its credit spread risk (spread DV01 measured against the OAS or option-adjusted spread). These risks behave differently in different market environments.
During credit events — such as the IL&FS default in 2018 or the DHFL crisis in 2019 — corporate bond spreads widened sharply across the credit quality spectrum while government bond yields remained relatively stable or even fell. Portfolio managers with high spread duration — who held large positions in long-maturity corporate bonds with wide spread exposure — suffered significant mark-to-market losses even though their interest rate duration risk had been carefully managed. This episode illustrated why spread duration is a critical independent risk metric in Indian fixed income portfolio management.
Spread duration is approximated for a bullet bond as approximately equal to modified duration. For a bond with a 7-year maturity and a 5% coupon trading at par, the modified duration is approximately 5.8 years. A 10 bps widening in credit spread translates to approximately 0.58% decline in the bond's price, holding the risk-free rate constant. For a corporate debt mutual fund with a portfolio spread duration of 4 years, a 25 bps widening in credit spreads across the portfolio would result in approximately 1% NAV decline from credit spread movement alone, separate from any impact from changes in G-Sec yields.
SEBI's monthly disclosure of portfolio YTM and modified duration for debt mutual funds gives investors a partial picture of interest rate risk, but spread duration is not separately disclosed. Sophisticated investors and advisors decompose the portfolio YTM into the G-Sec yield component and the credit spread component by comparing the fund's portfolio YTM against the corresponding maturity G-Sec yield, then multiplying the credit spread by an estimated average spread duration to assess credit spread risk exposure.