The following table contains information regarding tenant concentration in our portfolio, excluding $2.5 million for our corporate office and a credit loss reserve balance of $4.9 million, based on the percentage of revenues for the three months ended March 31, 2021 and 2020, related to tenants or affiliates of tenants, that exceed 10% of total revenue ($ in thousands):


	as of March 31, 2021						Revenues1
	Asset	Number of	Real		Notes		Three Months Ended March 31,
	Class	Properties	Estate		Receivable		2021			2020

Senior Living Communities	EFC	10	$	573,631	$	44,189	$	12,723	16%	$	12,717	15%
Bickford Senior Living	ALF	48	534,376		35,079		10,207		13%	13,603		16%
Holiday Retirement	ILF	26	532,672		—		10,185		13%	10,176		12%
National HealthCare Corporation (NHC)	SNF	42	171,235		—		9,452		12%	9,448		11%
All others2	Various		1,451,799		226,906		36,157		44%	35,579		42%
Escrow funds received from tenants
for property operating expenses	Various		—		—		2,161		2%	1,553		2%
			$	3,263,713	$	306,174	$	80,885		$	83,076

Under the terms of our two leases with NHC, rent escalates by 4% of the increase, if any, in each of the facility’s revenue over a base year and is referred to as percentage rent. The following table summarizes the percentage rent income from NHC ($ in thousands):


	Three Months Ended March 31,
		2021		2020
Current year		$	920	$	926
Prior year final certification1		(5)		(14)
Total percentage rent income		$	915	$	912

1 For purposes of the percentage rent calculation described in the master lease agreement, NHC’s annual revenue by facility for a given year is certified to NHI by March 31st of the following year.

Certain of our leases contain purchase options allowing tenants to acquire the leased properties. At March 31, 2021, we had a net investment of $40.2 million in six real estate properties which are subject to exercisable tenant purchase options. Tenant purchase options on 11 properties in which we had an aggregate net investment of $100.1 million at March 31, 2021, become exercisable between 2022 and 2028. Rental income from leased properties with tenant purchase options either currently exercisable or exercisable in the future was $5.0 million and $4.4 million for the three months ended March 31, 2021 and 2020, respectively.

In January 2021, we received notification of a tenant’s intention to acquire in July 2021, pursuant to a purchase option, one of the six properties mentioned above, a behavioral hospital located in Tennessee, for approximately $26.4 million. The net investment at March 31, 2021 was $21.1 million. Rental income was $0.7 million for both the three months ended March 31, 2021 and 2020. We cannot reasonably estimate at this time the probability that any other purchase options will be exercised in the future. Consideration to be received from the exercise of any tenant purchase option is expected to exceed our net investment in the leased property or properties.

Nine properties were transitioned during 2019 to five new tenants following a period of non-compliance by the former operators. Two leases with new tenants for six of these properties specify periods during which rental income is based on operating income, net of management fees. We recognized rental income from these nine properties of $0.9 million and $1.6 million for the three months ended March 31, 2021 and 2020, respectively.

We assess the collectability of lease payments to be received from our tenants, which includes receivables, consisting primarily of straight-line rents receivable, based on several factors, including payment history, the financial strength of the tenant and any guarantors, historical operations and operating trends of the property, and current economic conditions. If our evaluation of these factors indicates it is not probable that we will be able to collect substantially all of the lease payments, we recognize lease payments on a cash basis and de-recognize all rent receivable assets, including the straight-line rent receivable asset and record as a reduction in rental revenue.

Most of our existing leases contain annual escalators in rent payments. For financial statement purposes, rental income is recognized on a straight-line basis over the term of the lease where the lease contains fixed escalators. Some of our leases contain escalators that are determined annually based on a variable index or other factor that is indeterminable at the inception of the lease. The table below indicates the revenue recognized as a result of fixed and variable lease escalators ($ in thousands):


	Three Months Ended
	March 31,
		2021		2020
Lease payments based on fixed escalators, net of deferrals		$	67,281	$	68,669
Lease payments based on variable escalators		1,326		1,364
Straight-line rent income		4,241		5,177
Escrow funds received from tenants for property operating expenses		2,161		1,553
Amortization of lease incentives		(260)		(236)
Rental income		$	74,749	$	76,527

Effective April 30, 2021, we executed an agreement for the sale of six properties that were leased to Bickford for a purchase price of $52.9 million, which includes a $13.0 million Company-financed second mortgage. Upon completion of this transaction, Bickford satisfied the terms of our prior agreement that contingently waived $2.1 million in rent for the third quarter of 2020 and none of that amount will be repaid. These six properties had an aggregate net book value of approximately $34.6 million as of March 31, 2021. Rental income from this portfolio was $1.3 million and $1.5 million for the three months ended March 31, 2021