Frequency of Renal Monitoring — Creatinine and Cystatin C (FORM-2C): an observational cohort study of patients with reduced eGFR in primary care

Susannah Fleming; Rafael Perera-Salazar; Kathryn S Taylor; Louise Jones; FD Richard Hobbs; Tim James; Chris A O’Callaghan; Brian Shine; Jan Y Verbakel; Richard Stevens; Clare Bankhead

doi:10.3399/BJGP.2020.0940

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Figure 1.
Participant flow through the FORM-2C study. FORM-2C = Frequency of Renal Monitoring — Creatinine and Cystatin C.
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Figure 2.
Mean and 95% CIs for quartiles of a) eGFR-serum creatinine and b) eGFR-serum cystatin C calculated from the FORM-2C cohort at baseline and at 24 months.
CKD–EPI = Chronic Kidney Disease Epidemiology Collaboration. eGFR = estimated glomerular filtration rate. FORM-2C = Frequency of Renal Monitoring — Creatinine and Cystatin C.

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Mild chronic kidney disease is a common condition in primary care, but it is not clear what the optimal monitoring strategy should be. This research investigated whether serum creatinine or serum cystatin C is able to predict future decline in renal function in patients with mild chronic kidney disease. Although there is an indication that serum cystatin C-derived eGFR is more predictive of future renal decline in patients with eGFR <60 ml/min/1.73 m², there is insufficient evidence to suggest a change in practice. Clinicians should continue to choose an appropriate testing strategy for estimating current GFR, rather than considering the ability to predict future renal decline.

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Table 1.

Baseline characteristics of the FORM-2C population (n = 745)

Variable		Summary, n (%)^a	Missing, n (%)^a
Age, years, mean (SD)		69.8 (9.6)	0 (0.0)

Sex	Female	394 (52.9)	3 (0.4)
Sex	Male	348 (46.7)

Ethnicity	White	719 (96.5)	0 (0.0)
	Mixed ethnicity	6 (0.8)
	Asian	10 (1.3)
	Black	5 (0.7)
	Other	5 (0.7)

Comorbidities in GP record^b	Hypertension	429 (57.6)	n/a
	Chronic kidney disease	204 (27.4)
	Diabetes	164 (22.0)
	History of urinary tract infection	122 (16.4)
	Cancer	112 (15.0)
	Thyroid disease	89 (11.9)
	Prostate disorders	85 (11.4)
	Ischaemic heart disease	82 (11.0)
	Atrial fibrillation	54 (7.2)
	Myocardial infarction	44 (5.9)
	Kidney stones	40 (5.4)
	Transient ischaemic attack	28 (3.8)
	Stroke	19 (2.6)
	Heart failure	15 (2.0)
	Peripheral vascular disease	14 (1.9)

Smoking status	Never smoked	386 (51.8)	2 (0.3)
	Former smoker	327 (43.9)
	Current smoker	30 (4.0)

Biochemical markers at visit 1, mean (SD)	Serum creatinine (µmol/l)	88.4 (23.2)	16 (2.1)
	Serum cystatin C (mg/l)	1.2 (0.4)	83 (11.1)
	ACR (mg/mmol)	3.4 (9.4)^c	51 (6.8)
	eGFR-serum creatinine (ml/min/1.73 m²)	69.4 (16.2)	19 (2.6)
	eGFR-serum cystatin C (ml/min/1.73 m²)	61.9 (20.3)	86 (11.5)

eGFR over the extended baseline, mean (SD)	eGFR-serum creatinine (ml/min/1.73 m²)	68.9 (15.5)	6 (0.8)
eGFR over the extended baseline, mean (SD)	eGFR-serum cystatin C (ml/min/1.73 m²)	61.1 (19.4)	8 (1.1)

↵a Unless otherwise stated.
↵b Those with a diagnosis or history noted in the GP record at the first visit.
↵c ACR summaries do not include measures that were reported by the lab as ‘unable to calculate’ because of very low levels of urine albumin, or those entered as ‘normal’ on the GP system, where it was not possible to obtain a more accurate result. ACR = albumin:creatinine ratio. eGFR = estimated glomerular filtration rate. FORM-2C = Frequency of Renal Monitoring — Creatinine and Cystatin C. n/a = not applicable.

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Table 2.

Proportions of participants in the FORM-2C cohort (n = 745) classified into each eGFR range before the initial visit, at visit 1, and over the extended baseline (mean of values measured at visits 1–3)

eGFR range (ml/min/1.73 m²)	Pre-baseline eGFR, n (%)	eGFR-serum creatinine: visit 1, n (%)	eGFR-serum creatinine: extended baseline (visits 1–3), n (%)	eGFR-serum cystatin C: visit 1, n (%)	eGFR-serum cystatin C: extended baseline (visits 1–3), n (%)
≥90	7 (0.9)	63 (8.5)	47 (6.3)	68 (9.1)	63 (8.5)
≥60 and <90	520 (69.8)	462 (62.0)	487 (65.4)	279 (37.4)	320 (43.0)
≥45 and <60	167 (22.4)	136 (18.3)	144 (19.3)	176 (23.6)	200 (26.8)
≥30 and <45	49 (6.6)	58 (7.8)	57 (7.7)	100 (13.4)	118 (15.8)
<30	0 (0.0)	7 (0.9)	4 (0.5)	36 (4.8)	36 (4.8)
Missing	2 (0.3)	19 (2.6)	6 (0.8)	86 (11.5)	8 (1.1)

eGFR = estimated glomerular filtration rate. FORM-2C = Frequency of Renal Monitoring — Creatinine and Cystatin C.

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Table 3.

Results of primary analysis (n = 649) and secondary analyses in the FORM-2C cohort stratified by pre-baseline eGFR (n = 641), and separately stratified by age at baseline (n = 649)

Stratum	n	c-statistic for eGFR-serum creatinine (95% CI)	c-statistic for eGFR-serum cystatin C (95% CI)	Difference in c-statistics^a (95% CI)
All participants (primary analysis)	649	0.495 (0.471 to 0.519)	0.497 (0.468 to 0.525)	−0.00157 (−0.0358 to 0.0273)
Pre-baseline eGFR ≥60 and <90 ml/min/1.73 m²	450	0.492 (0.462 to 0.522)	0.530 (0.500 to 0.563)	−0.0379 (−0.0760 to 0.00289)
Pre-baseline eGFR ≥30 and <60 ml/min/1.73 m²	191	0.491 (0.442 to 0.540)	0.412 (0.371 to 0.469)	0.0789 (0.0178 to 0.145)
Aged<75 years	453	0.502 (0.473 to 0.529)	0.516 (0.484 to 0.546)	−0.0135 (−0.0492 to 0.0257)
Aged ≥75 years	196	0.496 (0.452 to 0.543)	0.448 (0.402 to 0.499)	0.0478 (−0.0135 to 0.109)

↵a Serum creatinine minus serum cystatin C. eGFR = estimated glomerular filtration rate. FORM-2C = Frequency of Renal Monitoring — Creatinine and Cystatin C.

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Frequency of Renal Monitoring — Creatinine and Cystatin C (FORM-2C): an observational cohort study of patients with reduced eGFR in primary care

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